The table represents a proportional relationship. Find the constant of proportionality and write an equation to represent the relationship.
$$
\begin{array}{|c|c|}
\hline a & y \\
\hline 2 & \frac{2}{3} \\
\hline 3 & 1 \\
\hline 10 & \frac{10}{3} \\
\hline 12 & 4 \\
\hline
\end{array}
$$
Constant of proportionality: ________
Equation: $y=$ ________