Output voltage for this OP-AMP circuit
As this is looks like homework these are just hints.

Let's call the voltage at the non inverting input of \$A_4\$  \$ x \$.

You already know that \$ A_1 \$, \$ A_2 \$ and \$ A_3 \$ are voltage followers so need to find \$ x \$.

From Kirchoff's current law we know that the sum of currents into any node is zero, i.e. what flows in must flow out, and the inputs to an op-amp take no current so.

$$ \dfrac{u_{11}-x}{R_1} + \dfrac{u_{12}-x}{R_1} + \dfrac{u_{13}-x}{R_1} + \dfrac{-x}{R_2} = 0$$

Can you find \$ x \$ from here?
Let's call the voltage at the inverting input of \$ A_4 \$ \$ y \$ what should it be?

Clue: \$ A_4 \$ has gain \$ G \$ and \$ G \$ can be taken to be very large.

The output voltage \$ u_o = G \cdot (x - y) \$ unless the amplifier saturates then it is limited by the supply rails.

What does \$ u_o \$ need to be to make this the case?