\documentclass[12pt, varwidth, border=5mm]{standalone}
\usepackage{tikz}
\usepackage{amsmath}
% Underlining package
\usepackage{ulem}
\usetikzlibrary{calc}
% \usepackage[a4paper, portrait, margin=1cm]{geometry}

\begin{document}
\section*{ }
    \begin{minipage}{0.55\textwidth}
  \begin{tikzpicture}[scale=1.5, baseline=(current bounding box.north)]

    \begin{scope}[rotate=34]
      \coordinate (A) at (0,0);
      \coordinate (B) at (5.875,0);
      \coordinate (C) at (intersection cs: first line={(A)--($(A)+(40:4cm)$)}, second line={(B)--($(B)+(180-35:4cm)$)});
      \draw (A) -- (B) -- (C) -- cycle;

      % Mark angles with arcs;
      \draw ($(A)!0.3cm!(B)$) arc [start angle=0, end angle=40, radius=0.3cm];
      \draw ($(B)!0.3cm!(C)$) arc [start angle=180-35, end angle=180, radius=0.3cm];
      \draw ($(C)!0.3cm!(A)$) arc [start angle=180+40, end angle=360-35, radius=0.3cm];

      % Label angles
      \node at ($(A)!-0.25cm!(B)$) {T};
      \node at ($(B)!-0.25cm!(C)$) {S};
      \node at ($(C)!-0.25cm!(A)$) {R};

      % Mark angles in degrees
      \coordinate (midBC) at ($(B)!0.5!(C)$);
      5\node at ($(A)!0.75cm!(midBC)$) {$40^\circ$};

      \coordinate (midAC) at ($(A)!0.5!(C)$);
      \node at ($(B)!0.75cm!(midAC)$) {$35^\circ$};

      \coordinate (midAB) at ($(A)!0.5!(B)$);
      \node at ($(C)!0.75cm!(midAB)$) {$\theta^\circ$};

    \end{scope}
  \end{tikzpicture}
\end{minipage}%
\hfill
\begin{minipage}{0.4\textwidth}
  \begin{align*}
    \text{$\theta^\circ$} &= 180^\circ - (\angle \text{T} + \angle \text{S}) \\
    &= 180^\circ - (40^\circ + 35^\circ) \\
    &= 180^\circ - 75^\circ \\
    &= 105^\circ
  \end{align*}
\end{minipage}

\end{document}
