To find the sum of S1, we use the formula for the sum of an arithmetic series:

S1 = n/2 $firstterm+lastterm$ S1 = 99/2 $1+99$ S1 = 4950

To find the sum of S2, we first find the number of terms in the series:

99 = 3 + $n-1$ * 3
99 = 3n
n = 33

Next, we use the formula for the sum of an arithmetic series again:

S2 = n/2 $firstterm+lastterm$ S2 = 33/2 $3+99$ S2 = 33/2 * 102
S2 = 1683

Therefore, s = S1 - S2 = 4950 - 1683 = 3267.