Question

A direct variation function contains the points (–9, –3) and (–12, –4). Which equation represents the function?
y = –3x
y = – x/3
y = x/3
y = 3x

Answer 1

First, find the slope between the two points. 
Slope = y2-y1 / x2-x1
-4+3/ -12+9
-1/-3
1/3 is the slope of this equation. 
You do not have to go any further to find b or the y-intercept because it appears that b or the y-intercept is 0 in this case. 
y= 1/3x or in other words y= x/3

Answer 2

(-9,-3)  (-12,-4)   use y2-y1/x2-x1=slope,   (-4+3)/(-12+9)=1/3    y=x/3