10.
$=\frac{1}{6}x(-8)y^{15}.(-9)x^5y=[(-8).\frac{1}{6}(-9)](x.x^5)(y^{15}y)=12x^6y^{16}$
11.
$=(-27)x^{12}y^{15}z^{18}.\frac{1}{9}x^5y^4$
$=(-27.\frac{1}{9})(x^{12}.x^5)(y^{15}.y^4)z^{18}$
$=-3x^{17}y^{19}z^{18}$
12.
$=2xy^2.\frac{1}{9}x^4y^6=(2.\frac{1}{9})(x.x^4)(y^2.y^6)=\frac{2}{9}x^5y^8$