The perimeter of an isosceles triangle is 20.4 m. Find its sides if the base is 3 m less than the lateral side.

Let’s designate the sides of the triangle (a and b) as x, then the base (c) will be equal to x – 3.
Through the perimeter, we find the length of the side x:
x + x + x – 3 = 20.4;
3x – 3 = 20.4;
3x = 20.4 + 3;
3x = 23.4;
x = 23.4 / 3;
x = 7.8 m.
The sides of the triangle a and b are 7.8 m each.
Find the base with:
c = x – 3 = 7.8 – 3 = 4.8 (m).
Checking: 7.8 + 7.8 + 4.8 = 20.4;
20.4 = 20.4.
Answer: a = 7.8 m, b = 7.8 m, c = 4.8 m.