The forces are,
[tex]F_1=F_2=5\text{ N}[/tex]The distance between the forces is equal to the diameter of the wheel.
The moment is,
[tex]\begin{gathered} 3=F_1\times d \\ d=\frac{3}{5} \\ d=0.6\text{ m} \end{gathered}[/tex]Hence, the diameter of the wheel is 0.6 m.