-2[tex]\sqrt{6x^{8}y^{3}}[/tex] * 4[tex]\sqrt{10xy^{8}}[/tex]
You can move [tex]x^{8}[/tex], [tex]y^{3}[/tex] and [tex]y^{8}[/tex] out of the square root to get
-2[tex]x^{4}[/tex][tex]y[/tex][tex]\sqrt{6y}[/tex] * 4[tex]y^{4}[/tex][tex]\sqrt{10x}[/tex]
Multiply them
-8[tex]x^{4}y^{5}\sqrt{60xy}[/tex]
You can take 4 out of 60 to get 2
-16[tex]x^{4}y^{5}\sqrt{15xy}[/tex]
