z^2 - z - 72
z^2 - 9z + 8z - 72
Step 1: 72 = 9 * 8 and when we subtract -9+8 = -1 and that's how I went from z^2 - z - 72 to z^2 - 9z + 8z - 72.
Step 2: Taking out common value from first and second; and third and fourth.
z^2 - 9z = z(z - 9)
8z - 72 = 8 (z - 9)
z^2 - 9x + 8x - 72
z(z-9) + 8(z-9)
(z-9) (z+8)
