Let us define the rate at which weekly sales are changing to be $\frac{dS}{dT}$
Let us define the rate at which weekly advertising costs are changing to be $\frac{dx}{dT}$
Let us define the change in weekly sales with respect to change in advertising costs to be $\frac{dS}{dx}$
$S=1980+45x+.65x^2$
$\frac{dS}{dT}$ = $\frac{dS}{dx}$*$\frac{dx}{dT}$
$\frac{dS}{dT}$ = $(45+1.3x)$*$\frac{dx}{dT}$
Now evaluate $\frac{dS}{dT}$
at x = $8400, $$\frac{dx}{dT}$ = 1250