Getting negative voltage on resistor?
Why do you think you are wrong?

Calculating Vo with V2 shorted will get you a negative voltage \$-2.5V\$ ... 

From \$ I*R2 = (V3 / (R1||R2 + R3) * R3 = (-4 / 3.2) * 2 = -2.5V\$

(Assuming the bottom line as your reference, which is traditional, and in keeping with the marked Vo.)

You can verify this in the simulator.







simulate this circuit &ndash; Schematic created using CircuitLab

That just means whatever \$V2\$ is, it needs to raise the output by \$4.5V\$.
Which should lead you nicely into the math of the second part of your superposition puzzle.