Question

In a certain arithmetic sequence, t6 + t15 = –141 and the common difference is –7.
What is the first term of the sequence?

explain how to do this please

Answer 1

you know t6 = a+5d
you know t15 = a+14d

so a+5d + a+14d = -141

don't you know d= -7 ???

take over ....