$$\small{\text{$
\begin{array}{lcl}
(4x^{2n-2}y^{4}-6x^{n}y^{n+3}+9x^{2}y^{2n+2})(2x^{n-1}y^{2}+3xy^{n+1})\\\\
=(4x^{2n-2}y^{4}-6x^{n}y^{n+3}+9x^{2}y^{2n+2})(2x^{n-1}y^{2})
+(4x^{2n-2}y^{4}-6x^{n}y^{n+3}+9x^{2}y^{2n+2})(3xy^{n+1})\\\\
=4x^{2n-2}y^{4}2x^{n-1}y^{2}
-6x^{n}y^{n+3}2x^{n-1}y^{2}
+9x^{2}y^{2n+2}2x^{n-1}y^{2}
+4x^{2n-2}y^{4}3xy^{n+1}
-6x^{n}y^{n+3}3xy^{n+1}
+9x^{2}y^{2n+2}3xy^{n+1}\\\\
=8x^{3n-3}y^{6}\textcolor[rgb]{1,0,0}{-12x^{2n-1}y^{n+5}}\textcolor[rgb]{0,0,1}{+18x^{n+1}y^{2n+4}}\textcolor[rgb]{1,0,0}{+12x^{2n-1}y^{n+5}}\textcolor[rgb]{0,0,1}{-18x^{n+1}y^{2n+4}}
+27x^{3}y^{3n+3}\\\\
=8x^{3n-3}y^{6}+27x^{3}y^{3n+3}\\\\
=y^3( 8x^{3n-3}y^{3}+27x^{3}y^{3n})\\\\
\end{array}
$}}$$
$$\small{\text{$
\begin{array}{lcl}
(4x^{2n-2}y^{4}-6x^{n}y^{n+3}+9x^{2}y^{2n+2})(2x^{n-1}y^{2}+3xy^{n+1})\\\\
=(4x^{2n-2}y^{4}-6x^{n}y^{n+3}+9x^{2}y^{2n+2})(2x^{n-1}y^{2})
+(4x^{2n-2}y^{4}-6x^{n}y^{n+3}+9x^{2}y^{2n+2})(3xy^{n+1})\\\\
=4x^{2n-2}y^{4}2x^{n-1}y^{2}
-6x^{n}y^{n+3}2x^{n-1}y^{2}
+9x^{2}y^{2n+2}2x^{n-1}y^{2}
+4x^{2n-2}y^{4}3xy^{n+1}
-6x^{n}y^{n+3}3xy^{n+1}
+9x^{2}y^{2n+2}3xy^{n+1}\\\\
=8x^{3n-3}y^{6}\textcolor[rgb]{1,0,0}{-12x^{2n-1}y^{n+5}}\textcolor[rgb]{0,0,1}{+18x^{n+1}y^{2n+4}}\textcolor[rgb]{1,0,0}{+12x^{2n-1}y^{n+5}}\textcolor[rgb]{0,0,1}{-18x^{n+1}y^{2n+4}}
+27x^{3}y^{3n+3}\\\\
=8x^{3n-3}y^{6}+27x^{3}y^{3n+3}\\\\
=y^3( 8x^{3n-3}y^{3}+27x^{3}y^{3n})\\\\
\end{array}
$}}$$