Since the 11th term is 78 and the difference is -9, that means when we go down terms we add 9 and if we go up terms we subtract 9. Since 11-1=10, we can multiply 9 by 10 to get 90 and add that to 78 to get 168 as our first term. Since we subtract 9 for every number from there, we can have 168-9(n-1). We have n-1 due to that the first term is 168, and we want to subtract 9 from each one. If we had it as 168-9n, the second term would be 168-18 when it should be 168-9. We can also add 9 to 168 to get 177 and have it as 177-9n due to that we start at the 0th term. This works due to that we are multiplying -9 by the nth value and adding that to 177
