slope-intercept form y=mx+b
to find the slope: (y2-y1)/(x2-x1)
(5-0) divided by (0-2.5)
(5) divided by (-2.5)
(-2) is the slope (m)
Then fill in with your point y=(-2)x+b
5=(-2)(0)+b and solve.
b= 5 to get y intercept value.
Then rewrite equation with your values
y=(-2)x+5