Math Help - What is the derivative of f(x) at this point?

1. What is the derivative of f(x) at this point?

(1)/(t)^2 at t=a. The answer, but more importantly, solving using this formula:

lim as t->a of [f(t)-f(a)] / t-a

$f(t)=\frac{1}{t^2}$.
$f(a)=\frac{1}{a^2}$.
$f'(a)=\lim_{t\to a} \frac{ \frac{1}{t^2} - \frac{1}{a^2} } { t-a }$.