Answer:
For an arithmetic series (note: the sum of the terms in an arithmetic sequence), the formula is:
Sn = ( 2a1 + (n-1)d ) * n/2
Plug in the values of:
n=25
a1 = -4
d = 3
S25 = ( 2*(-4) + (25-1)(3))*(25/2)
S25 = (-8 + 72)*25/2
S25 = (64)(25/2)
S25 = 800
