Finding the output voltage for op amp circuit
Here's how I'd do it: -



Once you have found the voltage at this junction (call it Vx) the problem is easy: -

\$U_A = V_x\frac{-R_4}{R_2}\$

How to find Vx? 

Maybe convert Ue and Ua to current sources like this: -



Then, the net current flowing through the parallel combination of R1, R2 and R3 is: -

\$\dfrac{U_E}{R_1} - \dfrac{U_A}{R_3}\$

That current multiplied by R1||R2||R3 is Vx

Can you take it from here?