(f•g)(10)

Answer 1

Without knowing the functions f and g, it is impossible to determine the value of (f•g)(10).

Answer 2

however, once you know f and g, then
(f•g)(10) = f(g(10))

Answer 3

Yes, that is correct. Once you know f and g, you can find (f•g)(10) by evaluating the composition of f and g at the input value of 10. This is equivalent to evaluating f at g(10).