In order to be able to write the equation, we first need to know what slope-intercept form is. Slope intercept form is written as: y=mx+b.
Your M represents your slope, Your x represents your x intercept, and your b represents your y intercept. Given the numbers in the problem, You would simply plug it in to follow each guide.
Your final answer is: y=m(-4.5)+(-3).
(Credit to whomever created the photo)