According to the Triangle Midsegment Thereom, if the midsegment of a triangle is parallel to a side of the triangle, then the midsegment is half the length of side it is parallel to, therefore...
45=(7x+13)/2
And after that, you continue to solve the problem using Algebra
90=7x+13
77=7x
11=x
I hope this answer was helpful! :)
