The property in question is the Transitive Property of Equality, which allows us to conclude that mLN equals 7 cm since mJK also equals 7 cm.
The property described in the question is the Transitive Property of Equality, which states that if one quantity equals a second quantity, and the second quantity equals a third quantity, then the first quantity equals the third quantity. In this case, because mJK equals 7 cm and mJK equals mLN, it follows that mLN also equals 7 cm.