What is the terminal velocity of a 6.00-kg mass object in falling with a drag force with a magnitude that depends on speed, v, as fdrag = (30.0 n∙s/m)v?

Gravitational acceleration assumed at 9.8 (m/s)/s 
Mass (m) = 30 kg 
If its still accelerating at 4 (m/s)/s at 70 m/s, then the net force on it = 30 * 4 = 120 Newtons 

Since the constant force due to gravity = 30 * 9.8 = 294 N , then the remaining force must represent the air drag at 70 m/s = 294 - 120 = 174 N 

The drag coefficient (Cd) can be derived from : 
Cd = 174 / velocity² 
Cd = 0.03551 

At terminal velocity, m * g = v² * Cd 
So, v = square root ( ( m * g ) / Cd ) 
v = 90.991 m/s (terminal velocity)