The first 4 terms of a different sequence are: 9, 13, 17, 21. Find an expression for the nth term of the sequence.

To tackle sequence questions like this you first have to find the difference between each term of the sequence and this number will go in front of the 'n'. Then to find the full expression, you have to find out the term that came before the first.

Applying this, the difference is 4 and the term came first would be 5. So the expression for the nth term is 4n+5