Find the Constant So That $$f ( x ) = \left\{ \begin{array} { l l } 0 & x < - 1 \\\frac { x ^ { 2 } } { 3 } & - 1 \leq x \leq A \\0 & x > A\end{array} \right.$$

Find the constant so that $$f ( x ) = \left\{ \begin{array} { l l } 0 & x < - 1 \\\frac { x ^ { 2 } } { 3 } & - 1 \leq x \leq A \\0 & x > A\end{array} \right.$$ is a probability density function.