In an isosceles triangle, two sides are the same. The perimeter of the sides is the sum of all the lengths of the sides. If x, y, and z are the lengths of the sides then x = y (since two sides have to be the same) and x + y + z = 2x + z = p where p is the perimeter.
So you have one side is 17cm less than twice the length of the other so this means s = 2*t - 17 algebraically. In an isosceles triangle, you have two sides that are the same so t = x and s = z, but we know that 2x + z = p = 95cm.
So given this information, can you re-arrange to solve for either x or z first and then plug this in to get the other value so you get three sides (remember: x,y,z are the lengths of the sides but x = y since we have an isosceles triangle)?