HOW TO SOLVE:
1) use x as a variable
2) the starting equation is:
          4x + 3x + 5x = 18
3) add all the like terms on the left side of the equation:
          12x = 18
4) Divide by 12 on both sides:
          x=1.5
5) Find the lengths of the sides:
        - 4x = 4(1.5) = 6
        - 3x = 3(1.5) =  4.5
       - 5x = 5(1.5) = 7.5
SO, the lengths of the sides of this triangle are: 6, 4.5, and 7.5