Question 861069
a, b, c, the side lengths of the triangle.
Translate the description:
b=2c;
a=3+b;
a+b+c=48.


Notice the first two equations both use b.  Suggestion is solve c in terms of b and solve a in terms of b (that one already given), and substitute into the perimeter equation.  Now you can solve for the value of b.  Solve then for the other two sides.