Hey there! I'm happy to help!
We want to find the equation of a line in y-intercept form, which is y=mx+b, where x and y are a point on the line, m is the slope, and b is the y-intercept.
We already know that our slope is -3.
y=-3x+b
We want to solve for b now. We can plug in our point (-9,-9) to solve for it.
-9=-3(-9)+b
-9=27+b
b=-36
So, our equation is y=-3x-36.
I hope that this helps! Have a wonderful day! :D