f = 3U/2
Explanation:[tex]U\text{ = }\frac{2}{3}f[/tex]cross multiply:
[tex]\begin{gathered} U\text{ = }\frac{2f}{3} \\ 3(U)\text{ = 2f} \end{gathered}[/tex]Divide both sides by 2:
[tex]\begin{gathered} \frac{3U\text{ }}{2}\text{= }\frac{\text{2f}}{2} \\ f\text{ = }\frac{3U\text{ }}{2} \end{gathered}[/tex]