The given electrical network is equivalent to

Output at NOR gate ${Y}_{1} = \overset{¯}{A + B}$

The output of first NOR gate is input to the second NOR gate, so the output of second NOR gate is ${Y}_{2} = \overset{¯}{{Y}_{1} + {Y}_{1}} = \overset{¯}{{Y}_{1}} = \overset{¯}{\overset{¯}{A + B}} = A + B$

The output of NOT gate is $Y = \overset{¯}{{Y}_{2}} = \overset{¯}{A + B}$

The given electrical network becomes equivalent to the NOR gate.