Functions and domain

[tourbound129]

Here is the entire question:

f(x)= 1/sqrt(28-x)

1. Find value of f for x=3
2. Find value of f for x=a+7
3. Determine domain of function: possible answers 0 less than or equal to x which is less than or equal to 28; x greater than or equal to 28; All Real numbers; or X>28

Here is what I have worked so far:
For #1 I came up with f(3)=1/5
For #2 I came up with f(a+7)= 1/sqrt(21-a) or 1/2*sqrt(-a)
For #3 I'm a bit thrown off because all 4 of the possible answers either include 28 (making the function invalid) or create a negative number in the denominator.

Any help is certainly appreciated.