Show us what you have tried.
I'm guessing that this function is defined only for an initial value onwards.
If this is the case, you need to substitute in your initial value into your function to obtain a value of c and show that the function has the right form.
So your initial value is at x = a with f(a) = b.
You can say that for this problem, you only define function from the initial value onwards to justify the zero before x = a.