The two congruent base angles tell us that this is an isosceles triangle, meaning the triangle has two congruent sides. Therefore, we can set these two expressions equal to each other and solve from there.
15x + 7 = 23x - 17
7 = 8x - 17
8x = 24
x = 3
Side = 15(3) + 7 = 45 + 7 = 52
Hope this helps!
