The following could represent aprobability density function(p.d.f.) or not. If could, find the value of k.

$\displaystyle

f(x) = \begin{cases}

k(x + x^2),

& \text {$-1 \le x \le 1,$} \\ 0, & \text {otherwise.} \end{cases}

$

How could I know it represent a p.d.f or not?