It's just coming up with a linear approximation for the function about some point, that is, finding the first-order Taylor polynomial of the function expanded about a given point.
Assuming you want to use a linear approximation of the function for some x near a, the linearization is
f(x) [itex]\approx[/itex] f(a) + f'(a)(x - a)
That's what vela was talking about when he mentioned the first-order Taylor polynomial. The "linear" part means that x appears to the first power only -- no higher powers.
BTW, this really seems like a calculus problem, so you probably should have posted it in the Calculus & Beyond section.