Solve the Problem $f ( x ) = 5 \sqrt { 2 x - 8 } - 5 \sqrt { x + 5 }$

Multiple Choice

Solve the problem.
-For the function $f ( x ) = 5 \sqrt { 2 x - 8 } - 5 \sqrt { x + 5 }$ , find all values $x$ for which $f ( x ) = - 15$ .

A) 4
B) 15
C) 9
D) $- 9$