Find the 90th term of the following sequence.
7 comma 14 comma 21 comma ...
The given sequence starts at 7 and increases by a difference of 7 each time. Therefore, the sequence is an arithmetic sequence with a common difference of 7.
To find the 90th term, we can use the formula for the nth term of an arithmetic sequence:
\[a_n = a_1 + (n-1)d\]
where $a_n$ is the nth term, $a_1$ is the first term, $n$ is the position of the term in the sequence, and $d$ is the common difference.
In this case, $a_1 = 7$, $n = 90$, and $d = 7$. Plugging these values into the formula, we have:
\[a_{90} = 7 + (90 - 1) \cdot 7 = 7 + 89 \cdot 7 = 7 + 623 = \boxed{630}\]
Therefore, the 90th term of the sequence is 630.