Let f(x)=x418x28\displaystyle {f{{\left({x}\right)}}}=\frac{{{x}^{{4}}-{18}{x}^{{2}}}}{{8}}.

(a)   Find
f(x)=\displaystyle {f}'{\left({x}\right)}=  

(b)   Is f\displaystyle {f} increasing or decreasing on (0, 3)?