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
2nd, y = y
= 18
3rd, z = y - 7
= 18 - 7
= 11