How can you say that there is nothing given for f(x)??
x^2-5 is continuous everywhere, including (-oo, c).
2x-6 is continuous everywhere, including [c, oo].
The only place where f(x) may be discontinuous is at x=c.
What would the value of c be to make the function continuous.
I would use the definition for a function to be continuous at x=c.
Can you state this definition (the answer has nothing to do with this given function)?