I need point/slope formula help!

Mathematics
Tutor: None Selected Time limit: 1 Day

I am preparing for the NY State Geometry Regent, and I answered a multiple-choice point/slope question. The question reads: What is an equation of the line that passes through (-9,12) and is perpendicular to the line whose equation is y = 1/3x + 6?

The correct answer was: y = -3x - 15.

I went over the math once again after seeing the correct answer. I still got y = 1/3x + 15. I don't see how the answer is y = -3x - 15. Can someone please explain the problem to me?

Jun 18th, 2015

Thank you for the opportunity to help you with your question!

if you want to test if lines are perpendicular  the product of two lines must be equal to negative 1

let the gradient of the second line be m

1/3 x m=-1

m=-3

general equation is y=mx+c

given point(-9,12)

12=-3 x -9+c

12=27+c

c=-15

y=-3x-15 

Please let me know if you need any clarification. I'm always happy to answer your questions.
Jun 18th, 2015

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Jun 18th, 2015
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