t6 + t15 = -141 and the common difference is -7. What is the first term of the sequence?

Answer 1

t6 + t15 = -141
t15 - t6 = -7*9 = -63
2 t15 = -204
t15 = -102
t1 = -102 + 14*7 = -4

The sequence is
-4, -11, -18, -25, -32,
-39, -46, -53, -60, -67,
-74, -81, -88, -95, -102

and the first term is -4.