-1x^2 + 4x + 21 = 0

-1( x^2- 4x - 21)=0

-1(x )(x ) ( -7)( +3)=0

-1(x-7) (x+3) = 0

Solve:

-1(x-7)=0

-1(x) (-1)(-7)=0

-1x+7=0

-1x+7-7=0-7

-1x=-7

-1x/-1=-7/-1

x=7

Solve:

-1 (x+3) = 0

-1 (x) (-1)(+3) = 0

-1x-3=0

-1x-3+3=0+3

-1x=3

-1x/-1=3/-1

x=-3