1/21 =1 / [F + 9D],
1/-10=1 /[F + 20D], solve for F,
Use substitution to get:
F ==510 / 11 - this is the 1st term
D== - 31 / 11 - this is the common difference
210th term ==[510 /11] + [- 31/11 * 209]
==[510 /11] + [- 6479 / 11]
== - 5969 / 11 - this is the 210th term
