If:
[tex]g(x) = x^2-x[/tex]
find g(3b). In this problem, you'd literally plug the 3b into the equation. Let's do that:
[tex]g(3b) = (3b)^2-3b[/tex]
[tex]g(3b) = 9b^2-3b[/tex]
We can further simplify this by pulling out a 3 and a "b":
[tex]g(3b) = 3b(3b-1)[/tex]
So, your answer is g(3b) = 3b(b-1).