The slope-intercept form:

m - slope
b - y-intercept → (0, b)
We have the point (0, 5). Therefore b = 5.
We have: y = mx + 5
Other point (3, 2). Substitute the coordinates of the point to the equation of the line:
subtract 5 from both sides
divide both sides by 3

Answer: f(x) = -1x + 5 = -x + 5