To find $ a_{ 27 } $ we use formula

$$ \color{blue}{a_n = a_1 + (n-1)d}$$

In this example we have $ a_1 = -21,~~ d = 7 ~~,~~ n = 27 $. After substituting these values into the formula, we obtain:

$$ \begin{aligned} a_n &= a_1 + d(n-1) \\[1 em] a_{ 27 } &= -21 + (27-1) \cdot 7 \\[1 em] a_{ 27 } &= -21 + 182 \\[1 em] a_{ 27 } &= 161 \end{aligned} $$

The first few terms of this sequence are:

$$ -21, ~~~-14, ~~~-7, ~~~0, ~~~7 . . . $$

Arithmetic sequences