Question 888275
S = ½(a + an)n
S = ½(2a + (n-1)d)n
an = a + (n - 1)d
  
We will use
S = ½(2a + (n-1)d)n
since we don't know the 15th term
S = ½(2*7 + (14)(-4))15
S = (7 + (7)(-4))15
S = (7 -28)15
S = (-21)15
S=-315