Maths help heeded

[3x (x - 2)] + [x(x - 4)]=36

4 x^2 - 10 x = 36
2x^2  -  5x  -  18 =0
Use the quadratic formula to find x:
x =4.5                    and x = -2 (discard)
Therefore, the length of AB = x = 4.5 cm