Answer: 90 cm
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Work Shown
L = unknown length
W = width = 57
P = perimeter of a rectangle = 2(L+W) = 294
2(L+W) = 294
2(L+57) = 294
L+57 = 294/2
L+57 = 147
L = 147-57
L = 90
Another way to solve would be this set of steps
2(L+57) = 294
2L+114 = 294
2L = 294-114
2L = 180
L = 180/2
L = 90
The length is 90 cm.
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Check:
P = 2(L+W)
P = 2(90+57)
P = 2(147)
P = 294
Or we could add up the four sides like this: 90+57+90+57 = 294
The answer is confirmed.