You need the outer and inner to sum to -2x. 3x * -2 = -6x 4 * x = 4x -6x + 4x = -2x You are basically just doing FOIL in reverse. This is known as factoring by trial and error or reverse FOIL.
You have: 1/(2x) + 2/(4x) = 12 First off, you can reduce the fraction 2/(4x) since 2/4 = 1/2 1/(2x) + 1/(2x) = 12 You can now change 1/(2x) + 1/(2x) into 2 * 1/(2x) 2 * 1/(2x) = 12 2/(2x) = 12 Simplify 2/2 = 1 1/x = 12 At this point, you have a lot of options. Easiest to understand is cross multiply: 1/x = 12/1 12x = 1 x = 1/12 Check: 1/(2 * 1/12) + 2/(4 * 1/12) = 12 1/(1/6) + 2/(1/3) = 12 6 + 6 = 12 12 = 12 This tells us that 1/12 is the correct solution.