The interquartile range of the data set is 4.

2, 2, 3, 3, 4, 5, 5, 6, 7, 9, 12

which explains whether or not 12 is an outlier?

twelve is an outlier because it is greater than the sum of 7 and 4.
twelve is an outlier because it is less than the sum of 7 and 6.
twelve is not an outlier because it is greater than the sum of 7 and 4.
twelve is not an outlier because it is less than the sum of 7 and 6.

Given: 
Interquartile range = 4.
Median: 5 ⇒ is also Q2.

Q1 = 3
Q3 = 7

IQR = IQ3 - IQ1 ⇒ 7 - 3 = 4
INNER FENCE
4 * 1.5 = 6 
7 + 6 = 13
4 - 6 = -2
Inner fence boundaries are -6 and 13.

OUTER FENCE
4 * 3 = 12
7 + 12 = 19
4 - 12 = -8
Outer fence boundaries are -8 and 19

Twelve is not an outlier because it is less than the sum of 7 and 6.

Outliers are number that are way out of the boundaries set in both the inner and outer fence. 

The linear equation is: y=ax+b