$a)\left(9a^{2}bc-36a{b}^3{c}^3\right):\left(3ac\right)=$   $3ab-12b^2{c}^2$  

$b)\left(49k^3{m}^5+140k^4{m}^4-84k^5{m}^3\right):\left(-7k^3{m}^3\right)=$