Given :
$l= 15 cm = 15 \times 10^{-2} m$
$B = 0.50 T$
$R = 9m \Omega = 9 \times 10^{-3} \Omega$
$v = 12 cm/s = 12 \times 10^{-2} m/s$
$\begin{align*} \text{The regarding force in the magnetic field is given by} F = BI l & = B . \frac{e}{R} . l \\ & = \frac{0.5 \times 9 \times 10^{-3} \times 15 \times 10^{-2}}{9 \times 10^{-3} } \\ & = 7.5 \times 10^{-2} N\end{align*}$