Which term of an AP : 21,

(b) Let nth term of the given AP be 210

Here, first term, $\quad a=21$

and common difference, $d=42-21=21$ and $a_{n}=210$

$\because$ $a_{n}=a+(n-1) d$

$\Rightarrow \quad 210=21+(n-1) 21$

$\Rightarrow \quad 210=21+21 n-21$

$\Rightarrow \quad 210=21 n \Rightarrow n=10$

Hence, the 10th term of an AP is 210.