-x^2+10x+21
Multiply out (x+3)^2 term;
(x+3)(x+3)
-x^2 + 3x + 3x + 9
Multiply out the 4(x+3) term;
4(x+3)
4x + 12
Collect similar real numbers, then x terms & then x^2 items together & hey presto
-x^2+10x + 21
Try putting the numbers into the quadratic formula, they will not be whole numbers, I think.