The temperature at the end of the day was 64 degrees. The temperature had increased twice during the day, once by 6 degrees and another time by 3 degrees.
Write an expression that shows what the temperature was at the start of the day?
Let x represent the temperature at the start of the day.
The temperature increased by 6 degrees and then by another 3 degrees.
Thus, the expression that shows what the temperature was at the start of the day is: x + 6 + 3.
Simplifying this expression gives: x + 9.
Therefore, the temperature at the start of the day can be represented by x + 9.
Let x be the temperature at the start of the day.
The first temperature increase was by 6 degrees, so x + 6 is the temperature after the first increase.
The second temperature increase was by 3 degrees, so (x + 6) + 3 = x + 9 is the temperature after the second increase.
The temperature at the end of the day was 64 degrees, so x + 9 = 64.
Thus, the expression that shows what the temperature was at the start of the day is x = 64 - 9 = 55 degrees. Answer: \boxed{55}.