\documentclass[varwidth=true, border=2pt]{standalone}
\usepackage{pgfplots}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
\begin{axis}[
legend pos=south east,
axis x line=middle,
axis y line=middle,
grid = major,
grid style={dashed, gray!30},
xmin= 0,
xmax= 12,
ymin=-10,
ymax= 10,
xlabel=$x$,
ylabel=$y$,
tick align=outside,
enlargelimits=true]
\addplot[domain=0:12, red, thick,samples=500] {1/3*x^1.5};
\addplot[domain=0:12, orange, thick,samples=500] {1*x^1.5};
\addplot[domain=0:12, blue, thick,samples=500] {2*x^1.5};
\addplot[domain=0:12, red, thick,samples=500] {-1/3*x^1.5};
\addplot[domain=0:12, orange, thick,samples=500] {-1*x^1.5};
\addplot[domain=0:12, blue, thick,samples=500] {-2*x^1.5};
\addlegendentry{$a=\frac{1}{3}$}
\addlegendentry{$a=1$}
\addlegendentry{$a=2$}
\end{axis}
\end{tikzpicture}
\end{document}