Incrementing Number Formula

Question

I am trying to find a formula to express the nth term, the pattern is as follows:

n = 4, 5, 6, 7

its corresponding values are as follows:

6, 10, 15, 21

I know this is not worded very well, if anyone can help it would be most appreciated

Answer 1

The pattern is as follows: $$a_{n-3}=\dfrac{n(n-1)}{2},a_0=6$$ Or alternately, $$a_{n-3}=a_{n-4}+(n-3),a_4=6$$