Answer:
y-y₁ = -3(x-x₁)
First of all, when you start an equation with a slope, since the question gave the slope -3, I'm not sure what your looking for, but I'll give you an example
Example:
y-y₁ = m(x-x₁)
m = -3
y₁ = ? - I'll just put a random # = -1
x₁ = ? - I'll just put a random # = 5
y-(-1) = (-3)(x-(5))
y+1 = -3x+15 - I'm guessing
