Find $\displaystyle \mu=E(X)$, $\displaystyle \sigma^2$ and then $\displaystyle \sigma$
Recall that:
$\displaystyle \mu=E(X)=\sum_{x} xP(x)$
$\displaystyle \sigma^2=E(X^2)-\left[E(X)\right]^2=\left[\sum_x xP(x) \right]-\mu^2$
$\displaystyle \sigma=\sqrt{E(X^2)-\left[E(X)\right]^2}=\sqrt{\left[\sum_x xP(x) \right]-\mu^2}$
Do you think that you can solve the problem now?
--Chris