We know that these two angles are equal to each other (There is the "congruent" sign) so we can set them equal to each other and solve for x
3x - 17 = 25 - 3x
(3x + 3x) - 17 = 25 + (-3x + 3x)
6x + (- 17 + 17) = 25 + 17
6x/6 = 42/6
x = 7
Hope this helped!