how to write Loops in Functions?

I have this Vector

Fx = [x(1)+2*x(2); x(1)+5*x(2)];

I want to take the first element of Fx_(x(1)+2*x(2))(also the second, third ,,,) and Put them instead of x(1),x(2),...

F=[x(1)+x(2); 3*x(1)-2*x(2)];

Answer 1

[Thorsten]

It is not entirely clear to me what you want to achieve.

If you want to use Fx instead of x, write

F=[Fx(1)+Fx(2); 3*Fx(1)-2*Fx(2)];