basic algebra

3x {(2x-y over 3) - (x-2y over 6)}

Do you mean 3x { [ 2x - (y/3)] - [x - (2y/6) ] } ?

= 3x { [ 2x - (y/3)] - 1 [x - (2y/6) ] } (remember that there is an invisible 1 )

= 3x { 2x - y/3 - x + 2y/6 }

=(3x/6) ( 12x - 2y - 6x + 2y )

=x/2 ( 6x )

= 3x ²