The perimeter of a rectangle is found by multiplying the length by 2 and the width by 2 and adding them together.
P = 2L + 2W
Filling in the information given we can find the width:
16.8 = 2(3.6) + 2W
16.8 = 7.2 + 2W
2w = 16.8 - 7.2
2w = 9.6
w = 9.6 / 2
w = 4.8 in.