Applied minima/maxima etc

I have a quiz tomorrow and im having a little bit of problem understanding how to work with these questions. I understand fine Dirivatives / etc but not these sepcific problems.

1) An open-topped box having square horizontal cross section is to hold 108 cubic Feet of grain when full. What demensions should it have if its total area (base plus four vertical sides) is to be as small as possible.

so a) we're looking for the area being minimized

and b) Im not sure what the function would be.

after i find the function, i know im supposed to:

differentiate the function and find the zeroes of the first derivative to identify the stationary points which are potential minimum or maximum and then use the second derivative test to classify each stationary point as min / max

2) Find the least area of a triangle in the first quadrant formed by the coordinate axes in a line through the point (3,8)

and 3) A box is to be made from a 10 inch by 16 inch sheet of metal by cutting equal squares out of the corners and bending up the flaps to form sides. Find the dimensions that maximize the volume

This time i know the vunction is V= x(10-2x)(16-2x) .. But i am not sure why thats the function basically.

If you have time to help, id very much appreciate it!

Ty!