you have to figure how to get from one term to the other. so in this case, to get from 8 to 3, you have to minus 5. and same again to get from 3 to -2, you again have to -5.
so the first past of the answer is -5n.
then after that, you have to figure out how to get from -5, to the first term of your sequence. so you have to add 13, in this case.