When we ask you the domain of f, you need to search for a forbidden value ?
for example you can't divide by zero, so assume x was equal to seven.
Then you would have f(7) = 7 / 7-7 = 7 / 0 this would be impossible because you can't divide by zero, yes ?
Other than that everything works, so the answer is c : all real values of x such that x ≠ 7
Good Luck