DO operations are multiply by -4 then add 15.
UNDO operations are subtract 15 then divide by -4:
Start with zero, subtract 15 to get -15. Now divide -15 by -4 to get 15/4, so this is the value of x.
x²-8x+15=(x-3)(x-5) making the roots 3 and 5.
x²-8x+20=0; x=(8±√64-80)/2=4±√-16/2=4±2i. Roots are complex 4+2i and 4-2i.
x³-4x²-4x+16=0=x²(x-4)-4(x-4)=(x²-4)(x-4)=(x-2)(x+2)(x-4) making the roots -2, 2, 4.