Given:

$\displaystyle f(x)=\left\{\begin{array}{cc}2,&\mbox{ if }

x\leq -1\\ ax+b,& \mbox{ if } \--1 < x < 3\\ -2, & \mbox{ if } x\geq 3\end{array}\right.$

I need to find the constants a and b such that the function is continous on the entire real number line. how can i do this ? (step by step is helpful)