Answer :
P = 2(L + W)
P = 84
L = 25
now sub
84 = 2(25 + W)
84 = 50 + 2W
84 - 50 = 2W
34 = 2W
34/2 = W
17 = W <=== the width is 17 cm
P = 84
L = 25
now sub
84 = 2(25 + W)
84 = 50 + 2W
84 - 50 = 2W
34 = 2W
34/2 = W
17 = W <=== the width is 17 cm