I need help looking at this problem.

I'm tasked with finding f(x) and some number, a. I think it's supposed to resemble but that's not leading me anywhere.

Isn't the f(x) in this case with 4 being a? I don't need the derivative, so I think I'm supposed to go backwards. I guess I'm hoping someone might be familiar with this type of problem and could toss me a tip. Thanks at any rate.