I am really stuck on a question..

(Q) Find x from the following:

34.56, x, 24, 20.

The asnwer is 28.80. I don't know how to get that. The two fomula you can use are Tn = a+(n-1)d or Sn= n/2 [2a+(n-1)d]

Thanks if you can show me how to do it! (Clapping)