Question 943687
In a triangle there are 3 sides so that would mean 3 variables.

"One side of triangle is 3 times the shortest side. The third side is 20 feet more than the shortest side." <--- That is what you said and because of this information we can label our variables.

Shortest side is x 
Second side is "3 times the shortest side" so it would be 3x
The third side is "20 feet more than the shortest side" so it would be x+20

We know that the perimeter or all of the sides add up to 90 so that would be your equation.

x+3x+x+20=90     (Simplify)
        5x+20=90     (Subtract)
              5x=70     (Divide)
                x=14

So your shortest side would be 14. 
The second side would be 14x3 which is 42.
The third side is 14+20 which is 34.

Add them up and they should equal 90.

Hope I helped.