Differential Op Amp queston
There are a lot of ways to solve this.  If you assume that the TL081 is ideal, you can take some short cuts.  If it is ideal, the current into the inverting and non-inverting inputs is 0.  For the non-inverting input, the voltage is just 5*30/44.  You recognize this because it is just a voltage divider.  Now you know the feedback is going to force the inverting input to have the same voltage, which is 3.41V.  So the current through R1 is (10-3.41)/12k.  But this same current has to flow through R2.  Since the current into the non-inverting is 0.  So now you know the voltage across R2 and the voltage at the non-inverting input.  You can take it from here.  Understanding what is going on will help you more than formulas.