Using quotient and chain rules for derivatives

Hey so I have a calculus assignment and im not wrapping my head around many of the concepts for derivatives.

My quotient rule question:

a) Use the definition of the derivative to find f'(x) for f(x) = (1/3x-4).

b) Use the result from a) to find the equation of the tangent line to f(x) at the point (2, 1/2 ).

For this I believe I have answered the first part, with

f(x)=(1/3x-4)

f'(x)=(f(x)*g'(x)-f'(x)g(x))/f(x)^2

=-3/(3x^2-24x+16)

I'm don't know what to do to continue to part be.

Thanks very much for any help you might be able to offer.