1. Find the equation of the given line from the graph.
2. Find the negative reciprocal of this line. Call this reciprocal M.
3. Recognize that the park entrance is at (0,4).
4. Using the point-slope formula for the equation of a straight line, write out the equation for the slope M and the point (0,4):
y - 4 = M(x - 0), or y - 4 = Mx, or y = Mx + 4.