Find the intervals in which the following function is strictly increasing or strictly decreasing.

$6-9x-x^2$

Find the intervals in which the following function is strictly increasing or strictly decreasing.$(x+1{)}^3(x-1{)}^3$

Find the intervals in which the following function is strictly increasing or strictly decreasing.

$x^3(x-2{)}^2$

Find the intervals in which the following function is  strictly increasing or strictly decreasing. $x{e}^x$

Find the intervals in which the following function is strictly increasing or strictly decreasing.

$x^3+3x^2-6x+12$