Recall that arithmetic sequences are defined by having a common difference. So if the first term is a and the common difference is d, the second term will be a+d, and the third term a+2d. To find the next term we just add d, right?
So we only have four terms, making our sequence a, a+d, a+2d, a+3d. We're given two relationships between the terms. Can you turn these relationships into equations?