The terms are in arithmetic progression, so lets call the difference between terms d, and the first term a. Then we are given that the first term and seventh term sum to 6, therefore,

a + (a + 6d) = 6 .

We are then given that the twentieth term is 56, so

a + 19d = 56 .

So now we have two equations in two unknowns, which we (you) can solve to find a and d. From there you should be able to complete the question.

Hope this helps.