The formula that you have to use is this y=Mx+b (m is the slope, x is just x, and b is the y-intercept)
What you have to find first is the y-intercept and as you can see the y-intercept of this line is -3 because is when the line intercept the y-axis and from that point you go up(+) 1 time and right(+) to times to reach the other point so the slope of the line is 1/2.
The slope form is m=rise/run
Rise:up(+)/down(-) and run:right(+)/left(-)
The equation of the line is y=1/2x-3
Hope this helps :D
A tip to know when the slope is going to be positive or not is when the line start and as you can see this line start from the bottom to the top so it positive