To find $ a_{ 34 } $ we use formula

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

In this example we have $ a_1 = 13,~~ d = 4 ~~,~~ n = 34 $. After substituting these values into the formula, we obtain:

$$ \begin{aligned} a_n &= a_1 + d(n-1) \\[1 em] a_{ 34 } &= 13 + (34-1) \cdot 4 \\[1 em] a_{ 34 } &= 13 + 132 \\[1 em] a_{ 34 } &= 145 \end{aligned} $$

The first few terms of this sequence are:

$$ 13, ~~~17, ~~~21, ~~~25, ~~~29 . . . $$

Arithmetic sequences