First we have to graph the area of interest
The area of interes is marked with purple color.
The vertices of such area are:
(0,0)
(0,3)
(2,1)
(3,0)
Now to find the minumum and maximum values of the function we evaluate the vertices in the function:
f(0,0)=0
f(0,3)=-3*3=-9
f(2,1)=-3(1)-4(2)=-11
f(3,0)=-4*3=-12
Finally we conclude that the maximum value is 0 and the minimum value is -12.