dQ/dt = Rate in - Rate out
The rate is is (x g/liter)*(2 liters/min) = 2x g/min.
The rate out is (Concentration)*(2 liters/min)
Now the concentration = Amount/Volume = Q/120
Thus, rate out is (Q/120 g/liter)(2 liters/min)=Q/60 g/min.
Thus, the differencial equation is,
dQ/dt = 2x - Q/60
dQ/dt + Q/60 = 2x
With initial conditions Q(0)=0.
This is a first order linear differencial equation, it solves easily. I am sure you can do it from there.