P = 2(l + w)
w = l/2
Plug in 'l/2' for 'w' in the equation with '66':
66 = 2(l + l/2)
66 = 2l + 2l/2
66 = 2l + l
66 = 3l
22 = l
So the length is 22.
Now plug this in to find the width.
66 = 2(22 + w)
66 = 44 + 2w
22 = 2w
11 = w
So the width is 11, and the length is 22.