Answer :
P = 2(L + W)
P = 84
L = 30
84 = 2(30 + W)
84 = 60 + 2W
84 - 60 = 2W
24 = 2W
24/2 = W
12 = W....width is 12 cm
P = 84
L = 30
84 = 2(30 + W)
84 = 60 + 2W
84 - 60 = 2W
24 = 2W
24/2 = W
12 = W....width is 12 cm