Rule:

If the ordered pair (a, b) is contained in a function F,
then F(a) = b

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(f∘g)(3) means 

Now since the ordered pair (3, 2) is contained in function g,
we know that g(3) = 2.

So (f∘g)(3) 

Now since the ordered pair (2, 3) is contained in function f,

we know that 

So altogether

 (f∘g)(3) 

Edwin