I presume you mean four terms of an arithmetic sequence rather than "four arithmetic sequence".
Surely your teacher has taught you what an "arithmetic sequence" is! An arithmetic sequence always has the property that the difference between two consecutive terms is the same. You are given the first two terms, a+ b and 2a- b. Their difference is (2a- b)- (a+ b)= a- 2b. The next term is given as 3a- 3b. The difference between the third and second terms is (3a- 3b)- (2a- b)= a- 2b also! So we know the "constant difference" is a- 2b. The fourth term must be the third term 3a- 3b plus that constant difference, 3a- 3b+ (a- 2b). What is that? Can you now find the fifth, sixth, seventh, and eighth terms? Finally, as a check add a- 2b to the eighth term to verify that you get 8a- 13b.