Identifying natural an forced response circuits
First step is to analyze the current flow of your circuit. You see that the capacitor is loaded by 2 volts and ends up at 9 volts.

Second step is the convert the current supply into a voltage supply; R stays 3 Ohm and the Voltage becomes 9v.
The charging formula of a resistor is:

Uc = U(1-e^(-t/τ)


While

τ = R*C = 3 Ohm * 2 F = 6s


The approach I would choose is with a time offset, just add extra time t0 to t, so that when time t is zero, the formula gives 2v

2V = 9V(1-e^(-t0/6s)) =&gt; t0=1.51s


Enter this in the formula:

Uc = 9V*(1-e^(-(t+1.51s)/6s))


And thats your function at the end. You can simplify this formula by splitting the e';

Uc = 9V(1-e^(-t/6s)*e^(-1.51/6))


And you end up by this; 

Uc = 9V - 7V*e^(-t/6s)