You could create a formula for it, as you have:
100 - 1st term, (100-8) - 2nd term, (100-8-8) - 3rd term.
The formula would be:
\(n=term\\ \\ V=value\\ \\ FORMULA:\\ \\ V=100-(n-1)8\)
Therefore, the value of the 50th term would be:
\(V=100-(50-1)8\\ \\ =100-49\cdot 8\\ \\ =-292\)