This is saying that ABC is conguent with LMN meaning all the sides are the same, making it safe to assume that AB is equal to LM. This also means that the equations for both are equal to each other
-3y + 10 = 2y + 15
-5y = 5
y = -1
Then to find AB you plug the -1 in for the y value
-3(-1) + 10 = 13
Knowing these 2 parts, the answer is A
