(Arithmetic Sequences)

How do I find K, given the consecutive arithmetic terms:

1. k, 8, k+11

2. k+2, 2k+3, 17

The only example we were given is:

-1 - (k+5) = (2k-1) - (-1)

-1 - k - 5 = 2k - 1 +1

-k -6 = 2k

-6 = 3k

k = -2

... which doesn't really apply, particularly through the first one.

I managed to figure out one:

31, k, 13

31-13=18

18/2=9

31-9=22

--> purely based on logic though, I don't know how to do the previous two.