Driven oscillations

A weight of mass  kg oscillates on a spring with a linear spring constant $k=$  N/m. The oscillations are damped with a linear damping coefficient $b =$  N s/m. A motor is used to force the weight to oscillate. After the motor has been on for a long time so that the transients have decayed, the motion of the weight can be described by,

What is the driving force exerted by the motor at time $t=$  s ?