## Word problem involving partial derivatives

So here's the problem from the top:

Biology A lake is to be stocked with smallmouth and largemouth bass. Let $x$ represent the number of smallmouth bass and let $y$ represent the number of largemouth bass in the lake. The weight of each fish is dependent on the population densities. After a six-month period, the weight of a single smallmouth bass is given by

$W1=3-0.002x-0.005y$

and the weight of a single largemouth bass is given by

$W2=4.5-0.003x-0.004y$.

Assuming that no fish die during the six-month period, how many smallmouth and largemouth bass should be stocked in the lake so that the total weight T of bass in the lake is a maximum?

Ok, so I'm not quite sure what to do with this problem. The rest of the section has been on taking first derivatives, finding critical points, and then using $d=f(xx) * f(yy) - f(xy)^2$ along with $f(xx)$ to determine if a specific point is a maximum, minimum, etc. I do know that it's looking for a maximum, so $d > 0$ must be true and $Fxx(a,b) < 0$ must also be true. Beyond that though, I'm not quite sure where to go...if I take the first partial derivative of either of those functions, I just get a constant...I'm assuming since it wants total weight that I do the same thing with each of the two equations and then add the answers together, however, I'm at a loss as to what specifically to do here.