You can put this solution on YOUR website! No that's NOT correct since you're dealing with inches and feet when everything should either be in inches only or feet only.
1 foot = 12 inches
So the length is 6 inches and the width is 12 inches
P = 2L + 2W
P = 2*6 + 2*12
P = 12 + 24
P = 36
So the perimeter is actually 36 inches (or 36/12 = 3 feet)