Let f(x)=(x-1)(x-2)(x-3)
Now, we can establish the sign chart
color(white)(aaaa)xcolor(white)(aaaa)-oocolor(white)(aaaaa)1color(white)(aaaaaa)2color(white)(aaaaaaa)3color(white)(aaaaaa)-oo
color(white)(aaaa)x-1color(white)(aaaaa)-color(white)(aaaaa)+color(white)(aaaaa)+color(white)(aaaaa)+
color(white)(aaaa)x-2color(white)(aaaaa)-color(white)(aaaaa)-color(white)(aaaaa)+color(white)(aaaaa)+
color(white)(aaaa)x-3color(white)(aaaaa)-color(white)(aaaaa)-color(white)(aaaaa)-color(white)(aaaaa)+
color(white)(aaaa)f(x)color(white)(aaaaaa)-color(white)(aaaaa)+color(white)(aaaaa)-color(white)(aaaaa)+
Therefore,
f(x)>=0 when x in [1 ,2 ] uu [3, +oo [