I am stumbling across this problem and was wondering if someone can help me, i have to find for what values of c and d will be differentiable for all values of x

$\displaystyle

f(x)=\left\{\begin{array}{cc}cx+d,&\mbox{ if }

x\leq -1\\cx^3+x+2d, & \mbox{ if } x>-1\end{array}\right.

$

thanks in advance