This is using seperation of variables (with a slight abuse of notation) we have:

dy/dx = x (6-y),

so:

dy/(6-y) = x dx

Now we could work with this but it is a bit more convienient to have the

coefficient of y in the denominator on the left to be +1, so we multiply

through by -1 to get:

dy/(y-6) = -x dx

RonL