Change in temp = -3/h
From noon to 1pm, 1 hour has passed, so temp = 0 degrees - 3 degrees = -3 degrees
From 1pm to 2pm, 1 hour has passed, so temp = -3 degrees - 3 degrees = -6 degrees
From 2pm to 3pm, 1 hour has passed, so temp = -6 degrees - 3 degrees = -9 degrees
From 3pm to 6pm, 3 hours have passed, so temp
= -9 degrees - (3 × 3 degrees)
= -9 degrees - 9 degrees = -18 degrees