I believe your answer is correct. The terminal velocity is when the force of gravity balances the force due to air resistance (and therefore acceleration is zero), and your calculations are correct. But you don't use a lot of the data in the problem (3000m, 20kg/s, 30s).
Could it be that the problem is (mis-)using the term "terminal velocity" to mean the velocity when the parachutist hits the ground? If so, then you need to solve a differential equation to get the position and velocity when the chute opens, then solve another (with k2 instead of k1) to find the velocity at the ground.
Post again if you're still having trouble.