This column, if we look at (x+p)(x+q) from right to left, will have x 2+px+qx+pq = x 2+(p+q) x+pq. If we look at it from the right, (x+3) (x+4) = x 2+7x. Or find the solution of the equation, and you will get the following formula. What you see from the left is the cross multiplication: x2+6x+913131*13 * 3 = 9. And 3+3=6. So x 2+6x+9 = (x+3) (x+3).