The way we would answer the question would be to express it as
100-9(n-1) with n being the nth term of the pattern.
Why is this? We know we subtract 9 each time and START from 100. Remember though that 100 is the first term in the sequence. So, we would have (n-1)*9 to subtract 0 from our 1st term and 9 from our second term.
Also, we have 9*(n-1) because we are just subtracting multiples of 9. It is always going to subtract by 9.
If you needed the answer here it is...
The answer to the problem would be 100-9(13-1) = 100-9(12) = 100-108 = -8
