Answer:
y = -
x+6
Explanation:
First off, you need to know what slope-intercept form is: y = mx+b
y = mx+b
b = the y-intercept, or where the line meets with (0,y), which is value of the coordinate is the y-intercept.
Looking at your yellow dot on the y axis, (0,6), could be the perfect fit to find the y-intercept.
So, know we know: y = mx+6
m = the slope of the line - you have to find two coordinates
in this case, it could be the two yellow points
(0,6)
(6,3)
use the following two coordinates to find the slope:

In this case, it would be 3-6/6-0, which equals to -1/2.
So, finally, we get:
y = -
x+6