the formula to find the perimeter of a rectangle is:
P = 2(l + w)
plug in your known values to solve for the unknown
20 = 2(l + 3)
20 = 2l + 6
14 = 2l
14/2 = l
7 = l or l = 7
you can check this by plugging your l value into the equation
P = 2(l + w)
20 = 2(7 + 3)
20 = 2(10)
20 = 20
hope this helps :)