Compute in symbolic math toolbox the result of function f(x) for x equals another symbolic expression

I have two functions defined with symbolic toolbox such as:

f(x)=a*x;
g(x)=b*x^2;

I want to set to x another symbolic expression.

which is:

x=k+1;

and the then find the difference:

g(k+1)-x(k+1)

Do you know how I do it?

Answer 1

[Giorgos Papakonstantinou]

Ok I found it.

dif=subs(g,x,k+1)-subs(f,x,k+1);