The function $f$ is defined by the formula $\qquad f(x) = \begin{cases} bx-4 &\text{if $x<-1$}\\ ax^2+b &\text{if $-1\leq x \leq 2$}\\ ax+1 &\text{if $x> 2$} \end{cases}$ **What values of $a$ and $b$ make $f$ continuous?**