Answer:
Step-by-step explanation:
This is part of the Isosceles Triangle Theorem. If one theta is equal to the other theta (and they are or else they wouldn't both be theta; one would be theta and the other might be beta), that means that the sides across from those congruent angles are also congruent. That means that
6x + 6 = 9x - 9 and
15 = 3x so
x = 5
