f(x) = 2x + x^2
g(x) = x+3
Now fg(x) means you're putting g into f, if it was fgh(x), you're putting h into g, then all that into f. Always right to left (just for convention, no real reason). Now algebraically, to solve this, you put g into f. How?
You're gonna take all of g, and wherever you see x, you're gonna replace it with all of g. So
fg(x) = 2(x+3) + (x+3)^2. All you've done is, replace x with whatever g(x) equalled. Brackets are important. Usually clean it up, to get the final marks.