\begin{tikzpicture}[vec/.style={-{Latex[length=3mm]}, thick}, >=Latex]

  % ---------- Painel 1: SPD (retas concorrentes) ----------
  \begin{scope}[xshift=0cm]
    \draw[->, gray] (-1.6,0) -- (1.6,0);
    \draw[->, gray] (0,-1.6) -- (0,1.6);
    \draw[thick, red!70!black] (-1.4,-1.0) -- (1.4,1.4) node[right, black] {$r_1$};
    \draw[thick, blue!70!black] (-1.4,1.2) -- (1.4,-0.8) node[right, black] {$r_2$};
    \filldraw[black] (0.13,0.35) circle (2pt);
    \node[below=0.5cm] at (0,-1.6) {\textbf{(a) SPD}: solução única};
  \end{scope}

  % ---------- Painel 2: SI (retas paralelas distintas) ----------
  \begin{scope}[xshift=5.5cm]
    \draw[->, gray] (-1.6,0) -- (1.6,0);
    \draw[->, gray] (0,-1.6) -- (0,1.6);
    \draw[thick, red!70!black] (-1.4,-1.0) -- (1.4,1.0) node[right, black] {$r_1$};
    \draw[thick, blue!70!black] (-1.4,-0.4) -- (1.4,1.6) node[right, black] {$r_2$};
    \node[below=0.5cm] at (0,-1.6) {\textbf{(b) SI}: nenhuma solução};
  \end{scope}

  % ---------- Painel 3: SPI (retas coincidentes) ----------
  \begin{scope}[xshift=11cm]
    \draw[->, gray] (-1.6,0) -- (1.6,0);
    \draw[->, gray] (0,-1.6) -- (0,1.6);
    \draw[thick, red!70!black] (-1.4,-1.0) -- (1.4,1.4);
    \draw[thick, dashed, blue!70!black] (-1.4,-1.0) -- (1.4,1.4);
    \node[right] at (1.4,1.4) {$r_1=r_2$};
    \node[below=0.5cm] at (0,-1.6) {\textbf{(c) SPI}: infinitas soluções};
  \end{scope}

\end{tikzpicture}