Mean
: The average of all the data in a set.
Median
: The value in a set which is most close to the middle of a range.
Mode
: The value which occures most frequently in a data set.
12,9,12,11,10,18,7,19,13,19
Mean = 13 (Average)
Median = 12 (Middle)
Mode = 12, 19 (most common)
Answer and Step-by-step explanation

2, -4, 6, -8, 10, -12, 14, -16
These are the next three numbers in the pattern :-12, 14, -16
3x^2 - 7x + 12 = 0 ....subtract 12 from both sides
3x^2 - 7x = -12 ....divide both sides by 3 to get x^2 by itself
x^2 - 7/3x = -4
His work is not accurate because he divided the second term by 4 instead of 3.