Then the force exerted by the liquid on the plate is approximately:

There will be backward viscous force acting on both the sides, first from liquid above the plate, and second from liquid below the plate. In this case $dx$ for both the plates is 3cm. So we have
$$\begin{gathered} F_v={\left(\mu A\frac{dv}{dx}\right)}_1+{\left(\mu A\frac{dv}{dx}\right)}_2 \\ \Rightarrow F_v=0.8\times 0.01\times \frac{1}{0.03}+0.8\times 0.01\times \frac{1}{0.03}=2\times 0.8\times 0.01\times \frac{1}{0.03}=0.53N \end{gathered}$$