I am looking at the following equation and its a little unclear what happened between steps 3 and 4, the last steps of factoring. How does [x+2] / [x+3] become [y-1]/ [y+1] ? Solution 3rd step :dy/dx = [y(x+2)-(x+2) ]/[y(x-3) +(x-3)] x+2 is common on nummerator and x-3 is common in denominator..so we took common we get..i.e 4th step dy/dx = (x+2)(y- 1)/(x-3) (y+1).