The notation is a little sketchy, but I think you mean f(x)*g(x), right?
We'll use the substitution property of equality. This states that for any x and y, if x = y, then we can use y in place of x, or x in place of y as needed.
Since f(x) = 2x + 5, we can replace f(x) with 2x + 5. We can also replace g(x) with 3x - 4.
To find f(x) * g(x), make the necessary replacements:
f(x) * g(x)
=(2x + 5)*(3x-4). Now FOIL it out:
=6x2 -8x + 15x - 20
=6x2 + 7x - 20. Right
