When something bisects an angle, it cuts that angle in half. This means that ray BD cuts angle ABC in half. Therefore, angle ABD, which is formed by cutting the angle in half, is equal to angle CBD, which is also formed by cutting the angle in half. Because of this, you have to set the equation equal to each other.
9x-18=5x+2
Combine like terms. Add 18 to both sides.
9x=5x+20
Combine like terms. Subtract 5x from both sides.
4x=20
Divide 4 on both sides.
x=5 degrees