The angle A is equal to B because they are opposite angles by the vertex.
So...
5x - 15 = 2x + 21
For solve it, we are going to do the same operations in both sides of the equal sign. We need put all the X in a side of the equal sign and all the free numbers in the other side.
5x - 15 = 2x + 21
We are going to add 15
5x - 15 + 15 = 2x + 21 + 15
5x = 2x + 36
We are going to substract 2x
5x - 2x = 2x + 36 - 2x
3x = 36
We are going to divide by 3
3x/3 = 36/3
x = 12
X = 12
and
B = A