Problem scenario
You read that there were two entrenchments in the U.S. Constitution. How do you know that they exist?
Answer
An entrenchment is the placement of something in a trench (according to Merriam-Webster's Dictionary). This connotes a defensible and protected area (e.g., for warfare). For a document constituting a government, an entrenchment is theoretically unmodifiable whereas the rest of the document is subject to being changed. For such a document, in practice, an entrenchment (an unamendable statement) is probably difficult to amend if the people or government want to amend it.
Article V of the Constitution explains the entrenchments. Article V describes amending the Constitution, and it specifies rules to bind three aspects of the Constitution from changes regardless of who may want to modify them in the future. Two of them, no longer apply because the year 1807 has passed; upon January 1, 1808, they ceased to have effect. One of these two was for the importation of slaves; the second of these two was for direct taxes on individuals. The third entrenchment is the guarantee of states having equal representation in the Senate (unless the state consents to waive it).
There were actually three entrenchments in the U.S. Constitution, but only one has applicability today. Both of the temporal (and now inactive) entrenchments pertained to the Legislative branch of government in the Constitution, and both were amended after the 1807 (via the 13th and 16th amendments respectively).
Here is Article V in its entirety:
The Congress, whenever two thirds of both Houses shall deem it necessary, shall propose Amendments to this Constitution, or, on the Application of the Legislatures of two thirds of the several States, shall call a Convention for proposing Amendments, which, in either Case, shall be valid to all Intents and Purposes, as Part of this Constitution, when ratified by the Legislatures of three fourths of the several States, or by Conventions in three fourths thereof, as the one or the other Mode of Ratification may be proposed by the Congress; Provided that no Amendment which may be made prior to the Year One thousand eight hundred and eight shall in any Manner affect the first and fourth Clauses in the Ninth Section of the first Article; and that no State, without its Consent, shall be deprived of its equal Suffrage in the Senate.
Taken from the U.S. Constitution
To read about Article I's section 9 clauses (the first and fourth thereof), see Congress' website. The 13th amendment (ratified on December 6, 1865) prohibited slavery in the U.S.A. (and thus modified the temporarily entrenched Article I's section 9's first clause). The 16th amendment (ratified on February 2, 1913) changed the Constitution's language for direct taxes on individuals (and thus modified the temporarily entrenched Article I's section 9's fourth clause).