So first we want to find how much bigger the triangle on the left is than the triangle on the right.
We know they are similar, because two of the angles are marked as the same, so therefore the third one must also be the same, which then means the triangles are similar.
51 / 3 = 17
So the triangle on the left is therefore 17 times bigger than the one on the right.
So what we can do is:
144 - x = 17x
Because if you multiply the lengths of the triangle on the right by 17, it'll be equal to the one on the left.
So here we can just solve the equation.
144 - x + x = 17 + x
144 = 18x
/18 /18
x = 8
So we know that x is equal to 8. Since AC's length is 144 - x, we can say it's also equal to 144 - 8. 144 - 8 = 136.
So your answer is C.
