Ya your teacher needs to tell you the coverage of a paint can. By that information, a can of paint could cover 349 square feet of room.

Damn I wish I was back at this level... *sigh*

Anyhow, for your program, you'll have something like this. (This is just pseudocode; don't copy it straight, because it won't work)

int totalarea
int doorarea
int windowarea
int paintablearea
int numdoors
int numwindows
int numcans

totalarea = 8 * 10 * 12

cout << how many doors?
cin >> numdoors
doorarea = numdoors * 3 * 7

cout << how many windows?
cin >> numwindows
windowarea = numwindows * 4 * 5

paintablearea = totalarea - (doorarea + windowarea)

numcans = ceil(paintablearea / WHATEVER A CAN PAINTS)
cout << The number of cans you'll need is << numcans

What is it they say about silence being golden?