Find x if triangle abc is congruent to triangle def

If triangle ABC is congruent to triangle DEF, the angles A and D are corresponding, angles B and E are corresponding, and angles C and F are corresponding. They are congruent as well.  Since the only measures we have for corresponding angles is angle B and E. Since they are congruent, we can set them equal to each other to solve for x:
3x + 8 = 4x - 7 and x = 15. We can't use A and D because we don't have a value for A to set equal to D.