First compute the area of the entire yard (L * W = A1). Then take the area of the house (HL * HW = A2). The yards area would then be A2 - A1 = YardArea.
If A1 = 5000 sq/ft and A2 = 2000 sq/ft then the yard's area would be (A2 - A1) or 3000 sq/ft.
If you can cut the grass at a rate of 2 sq ft a second then simply divide the yards area by 2. (i.e. 3000 / 2 = 1500 seconds or 25 minutes).
Yes, the specifics of the assignment will definitely help other experts assist you. So, its a C program. I can't write the program for you. However, you should have some text books (and internet) which should give you adequate instructions on obtaining values from user input (i.e. cin).
I apologize as I can't help much from this point forward. I'm no expert in C. My primary area of expertise is VB.Net. Good Luck.