To add or subtract terms with exponents, both the base and the exponent must be the same. In this case they are so subtract the coefficients and retain the base and exponent:

8c² - 9c²
(8 - 9)c²
-c²

The first few multiples of 3 are [3, 6, 9, 12, 15, 18, 21, 24, 27, 30] and the first few multiples of 5 are [5, 10, 15, 20, 25, 30, 35, 40, 45, 50]. The first few multiples they share are [15, 30, 45, 60, 75] making 15 the smallest multiple 3 and 5 have in common.

The factors of 16 are [1, 2, 4, 8, 16] and the factors of 64 are [1, 2, 4, 8, 16, 32, 64]. They share 5 factors [1, 2, 4, 8, 16] making 16 the greatest factor 16 and 64 have in common.

Average speed in miles per hour is the number of miles traveled divided by the number of hours:

Use PEMDAS (Parentheses, Exponents, Multipy/Divide, Add/Subtract):

5 + (3 + 3) ÷ 3 x 3 - 5²
P: 5 + (6) ÷ 3 x 3 - 5²
E: 5 + 6 ÷ 3 x 3 - 25
MD: 5 + \( \frac{6}{3} \) x 3 - 25
MD: 5 + \( \frac{18}{3} \) - 25
AS: \( \frac{15}{3} \) + \( \frac{18}{3} \) - 25
AS: \( \frac{33}{3} \) - 25
AS: \( \frac{33 - 75}{3} \)
\( \frac{-42}{3} \)
-14