oh ok, that was embarrassingly simple, I was totally fixed on the x, so I didn't want to put anything else than the x on the left side.

then i get

x = (xy-3y)/(y-5)

how do i get to

x= (3/5)y ?

yay, that's magic, thx.

well, that would explain, why it doesn't look like a linear eqsystem ^^

how do I solve it without maple?