-3,6, - 9,12, - 15,18, - 21. Expressed by the algebraic formula containing n

-3,6, - 9,12, - 15,18, - 21. Expressed by the algebraic formula containing n

Between positive and negative, the former multiplied by - 1 equals the coefficient of the latter
The absolute value is 3N
So the expression is: (- 1) ^ n × 3N, n = 1,2,3