Hello,
This is a composition problem. To solve problems like this we have to solve for the first expression (f(3) in this case) then plug the result of that into the 2nd function (g(x) in this case). So first let's find what

is. We know that

. Now let's substitute 3 in for x:

. Simplifying we get,

. So we know

. So now we need to find

. We know

is

. Now we can substitute 7 for x:

. Simplifying we get

. Therefore,

. If you have questions let me know ;-).