What’s the answer and how do i find it?

The arcs in the corners of the triangle signify that those two angles are equal. That means the triangle is isosceles, and the two sides opposite those angle are equal. In short,

... 4x +10 = 5x -25 . . . . . . sides opposite equal angles are equal

... 35 = x . . . . . . . . . . . . . . add 25-4x

entering that problem on the tube might get you an answer