Find the Constant $k$ Such That the Function $f ( x ) = k e ^ { - \frac { 9 } { 7 } x }$

Multiple Choice

Find the constant $k$ such that the function $f ( x ) = k e ^ { - \frac { 9 } { 7 } x }$ is a probability density function over the interval $[ 0 , \infty ]$ .

A) 1
B) $\frac { 7 } { 9 }$
C) $- \frac { 7 } { 9 }$
D) $- \frac { 9 } { 7 }$
E) $\frac { 9 } { 7 }$