As per given by the question,
There are given that function,

Now,
Find the value of f(g(3)).
Then,
There are given that,

And,
According to question, value of x is 3, that means g(3).
So,
Put the value of x in g(x).
Then,

Now,
For find the value f(g(3));
Put the value of g(3) in the above condition,
f(g(3)).
So,

Instead of x in f(x), put the g(3).
Then,

So, the value of f(g(x)) is 7.
Hence, the option first is correct.