The domain of a function is all values of x the function can assume.
Since the values inside a square root need to be non-negative and the values in a denominator can't be zero, we can write the following restrictions:
[tex]\begin{gathered} x-7\ge0 \\ x\ge7 \\ \\ x-4>0 \\ x>4 \end{gathered}[/tex]
The first condition is "inside" the second one, so we can use just the first one. Therefore, the domain is:
[tex]\lbrack7,\text{inf)}[/tex]
The square bracket in the beginning represents that number 7 is also included in the domain.