Given that $f(x)$ is continuous, answer the following questions using the information below:

$f^{\text{'}}(x)=(x-4{)}^2(x-10{)}^2(x+1{)}^5$

a$)$ Over what interval$($s$)$ is $f(x)$ increasing $($if none exist enter DNE$)$:

b$)$ Over what interval$($s$)$ is $f(x)$ decreasing $($if none exist enter DNE$)$: