The temperature on a certain morning is -11$°\mathrm{C}$ at 5 a. m. If the temperature drops 3 degrees at 6 a.m. and rises 5 degrees at 8 a.m. and again drops 3 degrees at 9 a.m. What is the temperature at 9 a.m.?

Temperature at 5 a.m. = –11$°\mathrm{C}$
Temperature decreased at 6 a.m. = 3$°\mathrm{C}$ = –3
Temperature raised at 8 a.m. = 5$°\mathrm{C}$ = + 5
Temperature decreased at 9 a.m. = 3$°\mathrm{C}$ = –3
Final temperature at 9 a.m. = (–11) + (–3) + (+5) + (–3)
= –11 – 3 + 5 – 3
= –17 + 5
= –12$°\mathrm{C}$