Question 121608
if a,b and c are the names of the sides (in decreasing order)
then
  a+b+c=40 (1st equation)
(perimeter is 40 cm)

a=b+c-4  (2nd equation)
(the length of the longest side is four cm less than the sum of th lengths of the other sides)

c=a-b+4 (3rd equation)
(The length of the shortest side is four cm more than the difference between the lengths of the other sides.)

Resuming:
  a+b+c=40 (1st equation)
  a=b+c-4  (2nd equation) then  b+c=a+4
  c=a-b+4  (3rd equation) then  b+c=a+4

look that 2nd and 3rd equations are the same thing

so we can not determine the sides b and c but.....
doing a substitution in the 1st equation we have:

a+(a+4)=40 so 2a+4=40 and a=18 

Then the length of the longest side is 18 cms