Answer
$y=1-2x^{2},\ \ x\in[-1,1]$
Work Step by Step
From the parametric equation for x, we see that $x\in[-1,1].$
Using a double angle identity formula for cosine,
$\cos 2t=1-2\sin^{2}t$
substituting, we arrive at $ \fbox{$y=1-2x^{2},\ \ x\in[-1,1]$}$
(part of a parabola that opens down)
To graph, create a table using several values for t, and then calculate $(x(t), y(t)),$ plotting the points as you go. Join with a smooth curve, noting the direction in which $t$ increases.
