Find the ordered pairs for the X and the Y intercepts of the equation 7x - 3y = 21.

I think the common difference is 2. The formula to create the sequence is n (number) is multiplied by 2.

1 x 2 = 2

2 x 2 = 4

4 x 2 = 8

8 x 2 = 16

16 x 2 = 32

32 x 2 = 64

64 x 2 = 128

128 x 2 = 256  

256 is the 8th term of the sequence. Since your question says that we should look for the sum, add the number of the sequence I guess.  

2 + 4 +8 + 16 + 32 + 64 + 128 + 256 = 510