width =w
length = 3w-4
P =2(l+w)
24 = 2(l+w)
substitute 3w-4 for l
24 = 2 (3w-4 +w)
24 = 2(4w-4)
distribute
24 = 8w-8
add 8 to each side
32 = 8w
divide by 8
w=4
l = 3w-4
l = 3*4-4
l=8
Answer: w = 4mm
l = 8mm
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