Answer
$\dfrac{3}{5}$
Work Step by Step
$\sin{t} = \dfrac{1}{\csc{t}} = \dfrac{\sqrt{5}}{5}$
$\cos{t} = - \sqrt{1- \sin^2{t}} =- \sqrt{1-\left(\dfrac{-\sqrt{5}}{5}\right)^2}$
$\cos{t} = \dfrac{-2\sqrt{5}}{5}$
$\cos{2t} = 1 -2 \sin^2{t} = 1 -2 \left( \dfrac{\sqrt{5}}{5}\right)^2 = \dfrac{3}{5}$
