A and B are alternate angles, so they have the same value as each other.
This means we make and solve an equation:
5x - 18 = 3x + 42
5x = 3x + 60 (add 18 to both sides to get 5x alone)
2x = 60
x = 30 degrees.Now that we know what x is, we can substitute it into the equation for A:
5x - 18 = <A
5 * 30 - 18 = <A
132 = <A
