Arectangle is 8cm long and has an area of 4g cm what is the width

The area of a rectangle is found by multiplying the length by the width. 
ie A = l x w 
given that the area is 40 square cm and one side is 8cm (I'll assume that it is the length) the formula becomes 
40 = 8 x w
Divide each side by 8 then 

40/8 = (8 x w)/8
5 = w
So the width must be 5 cm.
Perimeter = 2 x (l + w)
=2 x (8 + 5)
=2 x 13
=26 cm