Thank you!

Bliss, also note that the final line is invalid.

4xy + 20x - 12y does not become xy + 5x - 3y, since 4xy + 20x - 12y is an expression (rather than an equation). Factorisation would be good here.

4xy + 20x - 12y = 4 (xy + 5x - 3y)

In an equation, we can remove this “4” because the equation remains valid if we perform the same mathematical operation on both sides.

4xy + 20x - 12y = 0
4 (xy + 5x - 3y) = 0

Dividing both sides of the equation by 4,
xy + 5x - 3y = 0 (because 0 divided by 4 is 0)

More curiously, I am wondering how this could be a P6 level question - this is too advanced for a P6 student.

Exactly. I find this difficult for p6 & wondering if these kind of questions will be tested. Question source from tution center