A formula you can use to solve this problem is:
2x + (2x-7) = 17
The first 2x represents the two sides that have the same length, the (2x-7) represents the side that is 7m less than twice the common length, which altogether equal 17m, the perimeter.
From there, solve for x:
2x + (2x-7) = 17
4x - 7 = 17
4x = 24
x = 6
So two sides are 6m. The third side's length can be determined with 2x-7, so it is 5m.
