The perimeter is the sum of all sides. Start by listing out what the sides are given to be. Assign a variable to anything unknown.
1st side = x 2nd side = y 3rd side = z
Given: 1st side is 5 less than 2nd side. x = y - 5
Given: 3rd side is 2 less that 1st side. But we want everything in only one variable to solve. So use the first given 1st side = y - 5 z = y - 5 - 2 z = y - 7
Sum of sides (y - 5) + y + ( y - 7) = 42
3y - 12 = 42 3y = 42 + 12 3y = 54 y = 54/3 y = 18
Remember the sides, We have 1st, x = y - 5 = 18 - 5 = 13
