Calculus problem help (calc 1 AB)

I need some help with this Calc problem from Calc AB (calc 1)

A Tank with dimensions w x l x h has a rectangular base, rectangular sides and an open top. It is to be constructed so that its width is 4 meters and its volume is 36 cubic meters. Building the tank costs $10 per sqaure meter for the base and $5 per sqaure meter for the sides.

a) Write the cost function, C(h) in terms of h, where h is the height of the tank. What is the domain of C(h)

b)Find the critical points of C and analyze whther these critical points are the locations of extrema. Justify your answers.

c) What are the dimensions of the least expensive tank? Indicate units of measure.

d) What is the cost of the least expensive tank?

If you could help me that would be great, I've been staring at this for an hour, and for some reason I can't imagine how to do it.

Edit: Whoops, mod can move this to homework help if they like. Sorry.