+ + = 980 x^2 - 4x + 4 + x^2 + x^2 + 4x + 4 = 980 3x^2 + 8 = 980 3x^2 = 980 - 8 = 972 x^2 = 972/3 = 324 x = = +/- 18. ANSWER. There are two such sequences. One is 16, 18, 20. The other is -20, -18 and -16.