The term #3x^2# can be thought of as #3x*x# and the term #6x# can be thought of as #3x*2#. The #3x# is therefore a common factor that can be factored out (reversing the distributive property), and keeping the minus sign in place: #3x^2-6x=3x(x-2)#.
This also implies that the roots (#x#-intercepts) of the function #f(x)=3x^2-6x=3x(x-2)# are #x=0# and #x=2#.