Question 1173311
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P = 2L + 2W

You are given L=12in and P=6L:

6L = 2L + 2W
4L = 2W
4*12 = 2W

W = 24in   


Check (calculate Perimeter both ways):

P = 2L + 2W = 2*12 + 2*24 = 24 + 48 = 72
P = 6L = 6*12 = 72  (ok)