Question 884814
If you know the first and last terms use this
S = ½(A1 + An)n
If you know the first term and the difference between terms use this

S = ½(2*A1 + (n-1)d)n

We don;t know the last term
d=-6
n=100
A1=6

S = ½(2*(-6) + (99)*(-6))100

S = 50(-12 + (99)*(-6))

S = 50(-12-594)
S=50*-606
S=-30300