$\left\{ {\begin{array}{*{20}{c}}
{f(2x - 1) = 4{x^2} - 1}\\
{f(\frac{1}{2}) = ?\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,}
\end{array}} \right.$
$\begin{array}{l}
2x - 1 = \frac{1}{2} \Rightarrow 2x = \frac{3}{2} \Rightarrow x = \frac{3}{4}\\
f(\frac{1}{2}) = 4 \times {(\frac{3}{4})^2} - 1 = 1/25
\end{array}$