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Explanation:
L = length
W = width
"The length is 4 1/4 longer than the width" (paraphrase) so
L = W + 4 1/4
This means,
P = perimeter = 124 1/2
P = 2L+2W
2L+2W = P
2W+2L = P
2W+2( L ) = P
2W+2( W+4 1/4) = 124 1/2