Which term of the $AP: 21, 42, 63, 84, \ldots$ is $210$?

Find the $16^{th}$ term and the $n^{th}$ term of the $A.P.$ $9, 12, 15, \ldots$

$A$ manufacturer of $TV$ sets produced $600$ sets in the third year and $700$ sets in the seventh year. Assuming that the production increases uniformly by a fixed number every year,find the production in the $1^{st}$ year.

Two $AP$s have the same common difference. The first term of one of these is $-1$ and that of the other is $-8$. Then the difference between their $4^{\text{th}}$ terms is

The $20^{th}$ term of an $A.P.$ is $40$ and the common difference is $2$. Then,its second term is $\ldots \ldots \ldots \ldots$

For a given $A.P.$,the first term is $-4$ and the common difference is $-5$. Then,the $12^{th}$ term of the $A.P.$ is $\ldots \ldots \ldots$.