Question 629137
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