jmc

The constant term of the product of two polynomials is just the product of the two constant terms. Therefore we know that $6=-2a$, so $a=-3$. We now consider the linear term of the product of our polynomials. It's given by $-16t=(5t\cdot -2)+a\cdot bt⟹-16t=-10t+(-3)bt⟹b=2$. Therefore our answer is $a+b=\boxed{-1}$.