9514 1404 393
Answer:
-2018
Explanation:
The n-th term is ...
an = a1 +d(n -1)
So, the given terms are ...
-53 = a1 +12d
-128 = a1 +37d
Subtracting the first from the second gives ...
(a1 +37d) -(a1 +12d) = (-128) -(-53)
25d = -75
d = -3
The 668th term will be ...
a668 = a1 +d(668 -1) = a1 +667d = (a1 +37d) +630d
a668 = -128 +630(-3) = -128 -1890 . . . . substitute for a38
a668 = -2018