(6) if y = ax + b is the least square fit line for the points (−1, 3), (0, 2), (1, 4), find a + b.

We have the following function:
 y = ax + b
 We evaluate two points in the function:
 For (0, 2)
 2 = a (0) + b
 b = 2
 For (1, 4)
 4 = a (1) + b
 4 = a + 2
 a = 4-2
 a = 2
 Then, we have:
 a + b = 2 + 2 = 4
 
 The result of a + b equals 4