Write in slope-intercept form an equation of the line that passes through the given points. (−9, 5), (−3, 3)

The slope intercept form is the y = mx + b
We need to solve for the slope first
remember
the slope is the change in y/ over the change in x
y2 - y1 / x2 - x1
3 - 5 / -3 --9
-2/6 this can be simplified to -1/3
That is the slope so we plug it into the equation
y = mx + b
m = slope
y = -1/3x + b
Now to solve for the y-intercept, we plug in one of the coordinates to solve for b
3 = -1/3(-3) + b
3 = 1 + b
3 = b + 1
subtract 1 from both sides
2 = b
That is the point where it intercepts the y-axis
(0,2)

Now we have our equation for the line that passes through the points, (-9,5)(-3,3). 
That equation is y = -1/3x + 2

Hope this helps :)