According to postal regulations, the girth plus length of parcels sent by fourth class mail may not exceed 108 inches. what is the largest volume of a cylindrical parcel that can be sent by fourth-class mail?2r+1=108→Length=108-2r

I got:

V(d)= πr^2(108-2r)=108πr^2-2πr^3

V’(d)=216 πr-6 πr^2=0

r(36-r)=0

So , r=36, Length = 36

Is this Right?