Find the local linear approximation of the foloowing functions at the specified points:

a) f(x) = e^(5x) * cos(4x) at the point x=0

b) g(x) = e^(-5x) * ln(7x) at the point x=1

I know the basic idea of linear approx, where its f(x) + f'(x)*(x-a) but i have no idea what's what... for example is x or a equal to 0 in part a?