$f ( x ) = \left\{ \begin{array} { l l } 7 e ^ { - 7 x } & \text { for } x \geq 0 \\0 & \text { for } x < 0\end{array} \right.$

True/False

$f ( x ) = \left\{ \begin{array} { l l } 7 e ^ { - 7 x } & \text { for } x \geq 0 \\0 & \text { for } x < 0\end{array} \right.$ is a probability density function.