Solve the Problem $f ( x ) = - x ^ { 2 } + 4 x$

Multiple Choice

Solve the problem.
-Let $f ( x ) = - x ^ { 2 } + 4 x$ . Find all values $x$ for which $f ( x ) = - 21$ .

A) 4,0
B) $7 , - 3$
C) $- 7,3$
D) $\frac { 4 } { 21 } , \frac { 1 } { - 21 }$