Explanation:
so, we need to get the slope-intercept form
f(x) = y = ax + b
"a" is as always the slope, "b" is the y-intercept (the y-value when x = 0).
as we have the slope and a point, we can start with the point-slope form and transform it :
y - y1 = a(x - x1)
with (x1, y1) being a point on the line.
y - 7 = -9(x - 2) = -9x + 18
y = -9x + 25
therefore, the functional equation is
f(x) = -9x + 25