The perimeter of a rectangular garden is 80 feet, and the

Alternatively, you could just use the answer choices to solve this problem. Let's start in the middle, with answer choice C: If the "shorter" side is 34 feet, the "longer" side would have to be 6 feet in order to make the perimeter equal to 80 [(2 x longer side) + (2 x shorter side) = 80, so longer side + shorter side = 40.] . Clearly, that doesn't work. The "shorter" side would have be much smaller. Answer choice B is also too large because the shorter side being 23 would make the "longer" side 17. The only answer that seems reasonable is answer choice A. Short side = 17, Long side therefore =23. And 17 x 23 is indeed 391. All done.
Often, using real numbers is easier than algebra. So if you feel like you want to use algebra to solve a problem, and there are integers in the answer choices, tell yourself that one of those answers has to be the right one, so testing them out will get you the answer with less stress.