The function a(b) relates the area of a trapezoid with a given height of 12 and one base length of 9 with the length of its other base

Solving for b, we have the equation A(b)=12*(b+9)/2 - we cancel out 12 and the 1/2 to get 6(b+9)=A(b) and get 6b+54=A(b). Subtract 54 from both sides to get 6b=A(b)-54 and divide both sides by 6 to get b=A(b)/6-9 = A

10.4 is the answer for you