You might get asked to find the nth term of a sequence on numbers like:
5, 8, 11, 14, 17, 20
If they go up by a set number each time then this is easy. Simply multiply n by the difference each time, then work out what you need to add or subtract.
So for the first term in the sequence n=1 and here the difference between each number is 3.
3n is 3 so you need to add 2 to get to the first term (5).
So the equation is 3n + 2