to find x we need to know that lines DC and AB are parallel, so they have the same length (I don't know if that is why they have the same length, but I know they do lol)
So you can make this equation and solve:
15=x+3
-3 -3
12=x
THEN, to find y angle A and C are vertical angles, so they are congruent (this is the reason why they have the same measure lol)