$a)\frac{2}{3}x+\frac{5}{6}x-\frac{3}{2}x=$   $\frac{4}{6}x+\frac{5}{6}x-\frac{9}{6}x=$   $\frac{9}{6}x-\frac{9}{6}x=0$  

$b)\sqrt{81}{x}^{2}y-\sqrt{100}{x}^{2}y+\sqrt{49}{x}^{2}y=$