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Use the formula for summing up an arithmetic sequence: 15/2 * [2*7 + 14D] = - 210, solve for D 7.5 * [14 + 14D] = - 210 105 + 105D = -210 105D = - 210 - 105 105D = - 315 D = - 315 / 105 D = - 3 the common difference