#### Answer

$f'(t)= -7t^{-2}+15t^{-4} $

#### Work Step by Step

In order to derivate this function you have to apply the power rule:
$\dfrac{d}{dx}(x^n)=nx^{n-1}$
But first let's rewrite the function
$f(t) = 7t^{-1}-5t^{-3}$
Then:
$f'(t)= 7(-1)t^{-2}-5(-3)t^{-4} $
$f'(t)= -7t^{-2}+15t^{-4} $