for (13) you can solve for h in your second equation to get and then plug that into the formula for volume

you can go to wolframalpha.com and type in

Plot[Pi r^2(10-r/3),{r,0,31}] and play with the range of r to find your maximum.

do the same thing for (14), just solve for y, and plug that into the other equation.

and plot it similar to above.