Using a horizontal force of 200 N, we intend to move a wooden cabinet across a floor at a constant velocity. What is the frictional force that will be exerted on the cabinet?

For an object to move with constant velocity, net external force experienced by it must be equal to zero. Hence  F_external​ = 0 

which means that, F+ f = 0

where 'F' is the horizontal force and 'f' is the frictional force.

Hence f = -F = - 200 N

So a frictional force of -200 N will be exerted on the cabinet in the opposite direction so that it moves with a constant velocity under the applied horizontal force of 200 N.