평행선과 각의 크기

문제

다음 그림에서 두 직선 $l$과 $m$이 서로 평행할 때, $\angle x$의 크기는? (단, 점선은 한 직선을 나타낸다.)

\begin{center}
\begin{tikzpicture}[scale=1]
  % Parallel lines
  \draw (-2,0) -- (3,0) node[right] {$l$};
  \draw (-2,-2) -- (3,-2) node[right] {$m$};

  % Transversal line
  \draw (-1,1) -- (2.5,-3);

  % Intersections
  \coordinate (I1) at (intersection of -1,1--2.5,-3 and -2,0--3,0);
  \coordinate (I2) at (intersection of -1,1--2.5,-3 and -2,-2--3,-2);

  % Angle 120 degrees (exterior, top-left)
  \pic [draw, angle radius=7mm, "$\\!120^\circ$"] {angle = {(I1)+(-0.5,0.5)--(I1)--(I1)+(-0.5,0)}};

  % Angle x (interior, bottom-right)
  \pic [draw, angle radius=7mm, "$x$"] {angle = {(I2)+(0.5,0)--(I2)--(I2)+(0.5,-0.5)}};

\end{tikzpicture}
\end{center}