The trick here is realizing that both angles equal each other. There's a rule for this, but generally you can tell just by looking.
So what we can do is set these two equations equal each other, solve for x, and then plug x into angle B's original equation.
8x - 10 = 3x + 90
5x = 100
x = 20
now we plug that x into angle B
3x + 90
3(20) + 90
60 + 90
150
Angle B = 150°
