The given expression is ,
[tex]\frac{1}{3}ab^2=6[/tex]Divide both sides by 1/3 implies,
[tex]\begin{gathered} ab^2=\frac{6}{\frac{1}{3}} \\ ab^2=18 \end{gathered}[/tex]The, divide both sides by b^2 gives,
[tex]a=\frac{18}{b^2}[/tex]Therefore, a=18/b^2.