The profile of the velocity of fluid is as shown in the following figure

The velocity of the layer of fluid, which is in contact with metal plates (fixed), is zero. As we move towards the centre from either plate, the velocity of the layer of fluid increases, and it becomes maximum at the location of moving plate. This maximum value is same as that of the velocity of the plate.
The force experienced by the plate due to fluid is shown in the following figure.

F₁ is the force exerted by the layer in contact with plate on upper side and F₂ is the force exerted by the layer in contact with the plate on lower side.

${\mathrm{F}}_1=\mathrm{\eta A}\times \frac{\mathrm{v}}{4\mathrm{cm}}=0.8\times 0.01\times \frac{1}{0.04}=0.2\mathrm{N}$

Where v is the velocity of layer in contact with plate.

${\mathrm{F}}_2=\mathrm{\eta A}\times \frac{\mathrm{v}}{2\mathrm{cm}}=0.8\times 0.01\times \frac{1}{0.02}=0.2\mathrm{N}=0.4\mathrm{N}$

Total force, F = F₁ + F₂ = 0.6 N in backward direction.