Is this multiple choice or write in? The strategy for each type of problem is different.
If multiple choice and if the answers are numbers, then plug in answers and see which work. Remember the formula for area of a rectangle is LW=A and for Perimeter is 2L + 2W = P.
If it's not multiple choice then solve the second equation for L in terms of W, plug it into the first equation, and solve for W.
2L = 46-2W thus L = 23-W
(23-W)W = 76 thus
and 
Use the quadratic equation or the guess and check to find that quadratic factors to (W-19)(W-4) so W=19, 4.
L= 4 or L =19.
Since the question asks for the length of the shorter sides, the answer is 4.
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