I think it's actually a different answer. You wouldn't set the two equal to each other because they equal different angles, which both add up to 180 degrees (the angle measure of a straight line).
Instead, do...
(6x+15) + (3x+21) = 180
9x+36=180
9x=144
X=16
This makes sense because if you plug 16 back in for x, the left angle equals 111 degrees (an obtuse angle) and the right angle equals 69 degrees (an acute angle). 111 + 69 = 180 degrees. Hope this helps!!!