Calculating voltage at non-inverting input of the op-amp
The inverting input is not connected to ground- rather it is connected to a voltage divider between the op-amp output and ground. The output voltage is thus quite easy to find, given the voltage at the inverting input. But you don't know that voltage yet. 

When the op-amp is balanced, the voltage at the two inputs is equal. Ideally, the inputs don't affect the voltages. Ideally the output voltage is driven to whatever voltage balances the op-amp, assuming negative feedback and infinite gain.  

The voltage at the non-inverting input is just the mid-point of another voltage divider and the instructor has made it easier on you by setting one end to zero.