Non inverting amplifier

If Rf short circuited, Vo itzzt 6V

Do you know the answer now?
+input=6V
What is Vo?

As you know, in this case, the gain, Gv = V(out)/V(in+), is:
Gv = 1 + R2/R1
where
R1 is the resistance between in- and ground
R2 is the resistance between in- and output

If R2 = 0 (shorted), Gv = ...
V(out) = V(in+) * Gv = ...


Another approach:
Due to the very high gain, Gopen = V(out) / [ V(in+) -V(in-) ], of an opamp, the difference voltage at its inputs, V(in+) - V(in-), is close to zero since V(out) is usually a few volts only.