Explanation:
I assume you need the equation in slope-intercept form :
y = ax + b
"a" is the slope, "b" is the y-intercept (the y-value when x = 0).
the slope is defined as ratio (y coordinate change / x coordinate change) when going from one point on the line to another.
let's use the 2 points, where the line intersects the x- and the y- coordinate axis : (0, 9) and (9, 0)
that gives us "b" already : the y-value when x = 0 is 9.
about the slope "a" :
x changes by +9 (from 0 to 9).
y changes by -9 (from 9 to 0).
the slope is then -9/+9 = -1/1 = -1
and the equation is
y = -x + 9