Original function $\displaystyle \
R(t) = 2t^{\frac{{ - 1}}{2}}
\$
This differentiates to (if I'm not mistaken)
$\displaystyle \
- t^{\frac{{ - 3}}{2}}
\$
Bring the original power down and multiply it by the original coefficient and reduce the power by one to get the new power. It's a rule that just has to be learnt.