A rectangle has two pairs of sides of equal length.
So perimeter P = 2ft + 18in + 2ft + 18in
The units have to be all the same to add them so....
P = 2ft + 1.5ft + 2ft + 1.5 ft
P = 7 ft.
So the board would have to be 7 feet to make a 2ft x 1.5ft picture frame.