Could you please help me understand the work for this practice problem? Thanks!

A constant horizontal force of 18 N pushes a 40 kg-mass through a medium that resists its motion with 0.6 N for every m/s of speed. The initial velocity of the mass is 9 m/s in the direction opposite to the direction of the applied force. Find the velocity of the mass after 30 seconds. The differential equation is 40v'= 18 - 0.6v, v(0) = -9. Solving this gives v = 30 - 39e 30. Evaluating that at t=30 gives a velocity of 5.1325020867508 m/s m/s