Question

calc help please!!
this is about derivatives:
use the definition f'(a)=lim f(x)-f(a)/x-a
x-->a
to find the derivative of f(x)=(1/x) at x=3

so far, i have:
lim(1/x)-(1/a)/x-a
x-->a
what should i do next? should i plug in 3 for x in the numerator or should i take the LCD of the numerator to simplify it? help please!!!