Work backward. You should know the formula for finding the area of a triangle by now (Area = 1/2 base times height). So, with that given, use the possible choices they give you here.
If the height is 5 feet more than the base, and they give you the height, just choose one and work the problem out. Let's try the first one, using 10 as our height (so subtract 5 to get our base). 75=1/2(5x10) 75 = 50 is not a true statement. So let's try again, this time, use 15. 75 = 1/2 ( 10 x 15 ), Solve. 75 = 75
So, the answer here is 15.