The slope intercept formula is:
y = mx + b
m is the slope
b is the y-intercept
Let's first find slope:
The formula for slope is
In this case we have the points (-1, 5) and (2, -7)

^^^Plug these numbers into the formula for slope...

-4
^^^This is your slope
So far you have the equation:
y = -4x + b
Now we must find b
To do that you must plug in a point the line goes through in the x and y of the equation. This is the one I have chosen:
(-1, 5)
5 = -4(-1) + b
5 = 4 + b
1 = b
y = -4x + 1
Hope this helped!
~Just a girl in love with Shawn Mendes