Lets note the change between each of the numbers in the sequence:
-2 --> 6 ---> 14
Next, make an equation between each of these numbers with x being the change between them
-2+ x = 6, 6 + x = 14 .
Now, solve each of these equations:
-2+ x = 6 --> -2+ x = 6 Add 2 on both sides to cancel out the -2 --> x = 8
6+ x = 14 --> -2+ x = 6 Minus 6 on both sides to cancel out the 6 --> x = 8
From this, the common difference will be 8.
Hope that helps.
