begin{figure}[!h]
centering
caption{Gráfico (c)}
begin{tikzpicture}
    begin{axis}[xlabel = Entrada(n),ylabel = Tempo(s), samples at={50,55,60,...,145,150}]

        addplot[color=red,mark=x]coordinates {
            (50,        4.991e+000)
            (70,        2.704e+001)
            (100,       1.717e+002)
            (120,       4.696e+002)
            (140,       8.876e+002)
        };

        addplot[blue,thick]{(x)^(5)};

        legend{Estimado,Modelo}
    end{axis}
end{tikzpicture}
end{figure}

documentclass{article}
usepackage{pgfplots}
pgfplotsset{compat=1.12}
begin{document}
begin{tikzpicture}
    begin{axis}[xlabel = Entrada(n),ylabel = Tempo(s), %samples at={50,55,60,...,145,150}
    ]

        addplot[color=red,mark=x]coordinates {
            (50,        4.991e+000)
            (70,        2.704e+001)
            (100,       1.717e+002)
            (120,       4.696e+002)
            (140,       8.876e+002)
        };

        addplot[blue,thick,domain=50:140]{(x)^(5)};

        %legend{Estimado,Modelo}
    end{axis}
end{tikzpicture}
end{document}