Recall that arithmetic sequences are defined by having a common difference. So if the first term isaand the common difference isd, the second term will bea+d, and the third terma+2d. To find the next term we just addd, right?

So we only have four terms, making our sequencea,a+d,a+2d,a+3d. We're given two relationships between the terms. Can you turn these relationships into equations?