At 3pm, the outside temperature was 4.f. the temperature then fell steadily by 2.fevery hour for the next four hours. what was the temperture at 7pm.

4 - 2 - 2 - 2 - 2 = -4°F
It falls by 2 every hour. So starting with 4, you minus 2 for every hour that passes until you reach 7pm. 
OR
4 - (2 x 4) 
Start with 4, minus 2 four times (since it takes 4 hours to reach 7pm from 3pm)