Family of lines equation, need help
I'm super confused with this question: a family of lines have equations of the form 3x+by=12, where b is a non-zero number.
Find the equation of the line perpendicular to 3x+by=12 at the point (4,0).
I got the answer by=(1/3)x+12 but it says it's y=bx/3-4b/3.
Please help, thanks!