The first thing you have to find is the y-intercept.
The y-intercept is when the line INTERCEPT the y-axis and as you can see point 4 is the one that is intercepting.
To find the slope you just have to use the slope intercept form
M= rise/run
rise(up+/down-) and run(right+/left-)
So you put your pencil in point for and go 2 times up(+) and then 6 times left(-) to each the other point.
You slope is -2/6 or you can simplify to -1/3 because you can divide by 2 (2:2=1 and 6:2=3)
The slope was negative because number 2 is positive and number 6 is negative (+) + (-) is negative.
The final equation is y= -1/3x+4
The formula that you use is this y=Mx+b
(M is the slope, x is just x and b is the y intercept)
Hope this explanation helps you :D