If $f(x)=\frac{x+4}{x^2+ax+b}$, and $f(x)$ has two vertical asymptotes at $x=1$ and $x=-2$, find the sum of $a$ and $b$.
\((x-1)(x+2) = x^2 + x - 2\)
\(a + b = 1 - 2 = \boxed{-1}\)
+501 
