there are two ways you can do this. Go through each of the nth terms separately and find the one that matches or you can find the nth term with the sequence itself.
so you find the size difference inbetween each term which is -5 so you know there will be -5n in your formula so then you need to make n= 1 (for the first term) so that equals -5 but you want -3 so you need to find what + -5 equals -3 which is 2.
So now you know your answer is 2-5n