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
12x > 18x - 27 - 15 12x > 18x - 42 12x - 18x > -42 -6x > -42 x < -42/-6 two negatives make a positive so now its x < 42/6 Lastly divide 42/6 so it can equal 7
