\documentclass{article}
\usepackage{bm}
\begin{document}
\[
\bm{\left( \frac34 \right)^2}
\]

$$ \bm{\left(\frac34\right)^2} $$

$\displaystyle \bm{ p_n(x) = \left( \sum_{i=0}^n f[x_0,\cdots,x_i] \right)^2 }$

$\bm{\displaystyle p_n(x) = \left( \sum_{i=0}^n f[x_0,\cdots,x_i] \right)^2 }$
\end{document}
