In a triangle, the middle length side is 3 more than the shortest side, and the longest side is double the middle length side. the perimeter of the triangle is 45 cm.

Let's call the short side "s"
short side = "s"
middle side = 3s
longest side = 2*3*s
perimeter = 45 cm
perimeter = s + 3s + 6s = 45
short side = 4.5
middle = 13.5
long side = 27

4.5 + 13.5 + 27 = 45

The answer of the above is