The side of the longest triangle would be 9cm.
First, we need to find x
To do this, we add all 3 sides together to get the perimeter (20) :
x + x + 3 + x + 5 = 20
3x + 8 = 20
We then use this equation to find x :
3x + 8 = 20
(minus 8 on both sides)
3x = 12
(divide by 3 on both sides)
x = 4
Now we have x, we substitute it into the longest side (x + 5)
4 + 5 = 9cm.