Impliziertes Differenzieren mit y(x)

y soll alleine stehen, daher

3x+y(x) = 3   // -3x

y(x) = 3 -3x

I sit now for some time on the task: 3x + y (x) = 3

but not ferstehe why in the sample solution: 3 - 3x

comes out. Help would be very nice.

$3x + y (x) = 3\\  y (x) = 3-3x\\ y'(x)=-3\\$

OR

$3x+y(x)=3\\ 3+\frac{dy}{dx}=0\\ \frac{dy}{dx}=-3\\ $