In triangle ABC, AB = BC. If angle B contains x degrees, find the number of degrees in angle A

Answer 1

the three angles add up to 180°, so
B+A+C = 180
Now, AB=BC, so C = A.
Now just solve for A.

Answer 2

I see that a lot of you don't understand oobleck's response, so I'll elaborate on theirs. Since A = C, that means that you can substitute C in for A, and x for B. So:

A + B + C = 180
A + (x) + (A) = 180
2A + x = 180
2A = 180 - x
A = 180/2 - x/2
A = 90 - x/2