Question

find the equation of a line with an undefinded slope and that contains the point (9,−3)
A. y=−3
B. y=9
​C. x=9

Answer 1

any ideas?

Answer 2

Not at all

Answer 3

Can you help?

Answer 4

What do you know about slope?

Answer 5

the equation is y=mx+b

Answer 6

Ok, what about a definition?

Answer 7

Rise over run??

Answer 8

The slope of every horizontal line is 0.

A vertical line has no slope.
By "no slope" we mean the slope is undefined by the slope formula: $$\text{slope = } \dfrac{rise}{run}$$ ie when run = 0 you get a vertical line.

Answer 9

A line whose slope cannot be calculated, due to division by zero, is a vertical line.
This vertical line passes through the point (9, -3).

This is all the info you need with which to write the equation of this vertical line.