Let's rewrite the equation as
#x^2-8x<=0#, #=>#, #x(x-8)<=0#
Let #f(x)=x(x-8)#
Let's do a sign chart
#color(white)(aaaa)##x##color(white)(aaaa)##-oo##color(white)(aaaa)##0##color(white)(aaaa)##8##color(white)(aaaa)##+oo#
#color(white)(aaaa)##x##color(white)(aaaaaaaa)##-##color(white)(aaa)##+##color(white)(aaa)##+#
#color(white)(aaaa)##x-8##color(white)(aaaaa)##-##color(white)(aaa)##-##color(white)(aaa)##+#
#color(white)(aaaa)##f(x)##color(white)(aaaaaa)##+##color(white)(aaa)##-##color(white)(aaa)##+#
Therefore,
#f(x)<=0# when #x in [0 ,8 ] #, or #0<=x<=8#