Explanation:
since we can officially use only one point (the given point), we need to start with the point-slope form :
y - y1 = m(x - x1)
with m being the slope, (x1, y1) being the point.
sadly, to get the slope, we need a second point anyway, because the slope is the ratio
y coordinate difference / x coordinate difference
when going from one point on the line to another.
I recommend to use (0, 50) for the "other" point, as there seems to be the smallest error in the coordinates.
so, when going from (0, 50) to the given point (6, -40) :
x changes by +6 (from 0 to 6).
y changes by -90 (from 50 to -40).
the slope m = -90/+6 = -15/1 = -15
y - -40 = -15(x - 6) = -15x + 90
y + 40 = -15x + 90
y = -15x + 50
that is the slope-intercept form.
m = -15
b = 50
y = -242
-242 = -15x + 50
-292 = -15x
x = -292/-15 = 292/15 = 19.46666666... ≈ 19