Find the Next Term 21 , 15 , 9 , 3

This is an arithmetic sequence since there is a common difference between each term. In this case, adding ${\displaystyle -6}$ to the previous term in the sequence gives the next term. In other words, ${\displaystyle a_n=a_1+d(n-1)}$.

Arithmetic Sequence: ${\displaystyle d=-6}$

This is the formula of an arithmetic sequence.

Substitute in the values of ${\displaystyle a_1=21}$ and ${\displaystyle d=-6}$.

Add ${\displaystyle 21}$ and ${\displaystyle 6}$.

Substitute in the value of ${\displaystyle n}$ to find the ${\displaystyle n}$th term.

Multiply ${\displaystyle -6}$ by ${\displaystyle 5}$.