Solution (source code)

= Solution
{c}

The affine transformation
$$
\boxed{T(u)=2u-1}
$$
maps the <uniform distribution> on $(0,1)$ to the uniform distribution on $(-1,1)$. Indeed for $-1<v<1$, $\mathbb P(2U-1\leq v)=(v+1)/2$, whose derivative is the constant density $1/2$.