Semiconductor Electronics

The top gate is a NOR: $\overline{A+B}$. The bottom gate is a NAND: $\overline{A\cdot B}$. Their AND gives $Y=\bar{A}\bar{B}\cdot (\bar{A}+\bar{B})=\bar{A}\bar{B}$, which is exactly $\overline{A+B}$ — a NOR gate.