see here to see what the variables mean.
since the general term is given by , you have
and
you can solve this system for and and hence find
For an arithmetic sequence...
.
You're told that , so
and you're also told that , so
.
You now have 2 equations in 2 unknowns that you can solve simultaneously. Subtract equation 1 from equation 2 and you should find
.
Substitute back into equation 1 to get
.
Therefore .
What is ?