I am confused

I am trying to find a formula to link these numbers for a part of my GCSE coursework (i.e nth term) could someone please show me how i can do this??

1 2 3 4 5

3 6 10 15 21....

I know that the numbers underneath are triangle numbers, but i am confused about nth term, and dont know how to work this out.

so far i have done this:

1 2 3 4 5

3 6 10 15 21

3 4 5 6 - Difference

1 1 1 - Difference

But i do not know what these numbers mean? and how you can work out the nth term from it? (does the difference being incosistant then consistant show something? like n+1 or 1n or something? PLEASE HELP!!!)i want to know how i can get the answer! so please show me HOW you can find it!

HELP WOULD BE MUCH APPRECIATED!!!

Thanks

lilblob