Series parallel equivalency Rule :

Transistors in series $(W/L)_{eq}$ = $\Sigma_{K(on)}(L/W)$

Transistors in parallel $(W/L)_{K}$ = $\frac{I}{K_{(on)}}(W/L)_{K}$

$V_{th}(inv) = \frac{V_{T,n} + (\frac{K_P}{K_n})^{1/2}(V_{DD} - |V_{T,P}|)}{1 + (\frac{K_P}{K_n})^{1/2}}$

substitute, $K_P = K_P/2$ & $K_n = 2K_n$

$V_{Tn}(No) = \frac{V_{T,n} + (\frac{K_P}{4K_n})^{1/2}(V_{DD} - |V_{T,P}|)}{1 + (\frac{K_P}{4K_n})^{1/2}}$

To get $V_{Th}(NOR2) = \frac{V_{DD}}{2}, V_{T,n} = …