Can we combine these voltage sources?
Is it because of the ground between the two voltage sources that
  doesn't allow us to unite them?


That is correct. Because the ground was chosen there, the top-left voltage source will apply +3V on the top side of the \$91k\Omega\$, and the bottom voltage source will apply -3V on the bottom side of the \$150k\Omega\$. So you get the following equivalent schematic:







simulate this circuit &ndash; Schematic created using CircuitLab

Voltages are always relative, which is why we choose a "ground" node that we will refer to as 0V. You can abuse this property and choose a "new ground" node at -3V (between R2 and V2) by adding 3V to each node voltage in the circuit.

With this "new ground", you will get the solution that you calculated (3.73V). But you should not forget to then go back to the original "ground" reference, by decreasing each node voltage with 3V again, yielding 0.73V.