There are two resistors in parallel that can be replaced by a single resistor to make the circuit a little simpler. Then start from the right hand side and calculate voltages and currents from the information you have. Note for homework we will only try to help we will not give the answer.
One way is to collapse the network from right to left, with the goal to calculate v1 which is the juntion of 1.2 and 108 (and other resistors), calling the junction of 108 and 3 oihm resistors ground. The voltage you get will retain Vg as a variable.
Once you get v1, moving left to right you can calculate the voltage v2 which would be the junction of 5 ohms and 2 ohms, then v3 the junction of 3 ohms and 6 ohms, then the differential v2-v3, then you can calculate the voltage across the 12 and 6 ohm resistors (also the 9 ohm resistor) and that will be in terms of Vg such as:
K*Vg=Vout
Now since the current through the 12 and 6 ohm resistors is known and we can call it Iout, we know that dividing by the resistance gives us the current:
K*Vg=Vout
K*Vg/R=Vout/R
and of course the right side is the current 1 amp so:
K*Vg/R=1
and then solve for Vg, which is the required voltage.
So you have to figure out what R is and do the calculations. That's one way to do it.
The "collapsing right to left" works when all the elements can be easily combined along the way such as with this circuit. Another way is to use Nodal Analysis which is a general method for solving circuits.
I wont give the answer yet, but a range is between 50 and 80 volts and no it is not 65v either