First, let's define slope-intercept form: it's the form that's usually taught first in math with coordinates(at least in my experience):
y = mx + b, where m is the slope, and b is the y-intercept.
The problem already gives us a slope of -2, so we can already write part of our desired equation:
y = -2x + b
Next, the problem gives us a point of (-1, 8). Here, we can plug the point into the equation, with the respective x and y values, giving us:
8 = -2 * (-1) + b
This simplifies to
8 = 2 + b, or
b = 6
Finally, we have our desired equation, which is y = -2x + 6!(not factorial)