I'm assuming that this is really:
(40x - 3x^2)/ 3 = x(20-x)/ 3
This is a quadratic equation, not something you'll need to worry about in the General Course.
(40x – 3x2)/3 = x(20 – x)/3
Multiply both sides by 3.
40x – 3x2 = x(20 – x)
40x – 3x2 = 20x – x2
Add 3x2 to both sides.
40x = 20x + 2x2
Subtract 40x from both sides.
2x2 – 20x = 0
Factorise.
2x(x – 10) = 0
2x = 0 or x – 10 = 0
.: x = 0 or x = 10