f∘g(x) is read "f of (g of x)".

If means the same as 

To get it, substitute what g(x) equals, for x in what
f(x) equals.  Then simplify.

We start with f(x)



Then we take what g(x) equals, which is (4x+9), and
substitute it for x in , like this: 

So we have:

f∘g(x) = 

Then we simplify

f∘g(x) =  

f∘g(x) = 

That's the answer.

To find f∘g(5), we merely substitute 5 for x

f∘g(5) =  and simplify

f∘g(5) = 

f∘g(5) = 

Be sure to watch this youtube video:

https://www.youtube.com/watch?v=S4AEZElTPDo

Edwin