Let's rearrange the equation
#x^2<=8x#
#x^2-8x<=0#
#x(x-8)<=0#
Let #f(x)=x(x-8)#
Let's do the 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)(aaaaaaa)##-##color(white)(aaaa)##+##color(white)(aaaa)##+#
#color(white)(aaaa)##x-8##color(white)(aaaa)##-##color(white)(aaaa)##-##color(white)(aaaa)##+#
#color(white)(aaaa)##f(x)##color(white)(aaaaa)##+##color(white)(aaaa)##-##color(white)(aaaa)##+#
So, #f(x)<=0#, when # x in [0,8]#
graph{x(x-8) [-41.1, 41.1, -20.56, 20.56]}