You posted three of these. Look at this one, then try the other two.
They are done the same way.
Find slope: m = (change in y)/(change in x) = (4-(-5))/(-1-2)= 9/-3 = -3
In slope intercept form of y = mx + b where m is slope and b is y-intercept
y = -3x + b
Use one of the given points as (x,y) to solve for b
4 = -3(-1) + b
4 = 3 + b
1 = b
Your equation is: y = -3x + 1