We know that an isosceles triangle has 2 equal sides.
We can represent the equal sides with x, and the base with y:
2x + y = 15.6
Given states that y = x - 3, so:
2x + x - 3 = 15.6
3x = 18.6
x = 6.2
y = 3.2
The sides are 6.2 m, and the base is 3.2 m.
