Step-by-step explanation: For this problem lets have t be equal to the original temperature in the beginning of the day. The equation -9 + 21 = t represents this situation because it states that if we add the degrees that dropped to the current degrees it will give us our original degrees.
Solve.
-9 + 21 = t (Simply add -9 + 21) -9 + 21 = (+12)
12 = t
So, the original degrees in the beginning of the day was 12°C.
