Answer
a)
${{a}_{n}}=0.6n+17.8$.
b) $35.8 \%$.
Work Step by Step
(a)
The general equation of the nth term is given by ${{a}_{n}}={{a}_{1}}+\left( n-1 \right)d $
So, $\begin{align}
& {{a}_{n}}={{a}_{1}}+\left( n-1 \right)d \\
& {{a}_{n}}=18.4+\left( n-1 \right)0.6 \\
& {{a}_{n}}=18.4+0.6n-0.6 \\
& {{a}_{n}}=0.6n+17.8 \\
\end{align}$
(b)
Here, $ n=30$ because after 1989 until 2019, 30 years pass:
So, $\begin{align}
& {{a}_{n}}=0.6n+17.8 \\
& {{a}_{n}}=0.6\left( 30 \right)+17.8 \\
& {{a}_{n}}=35.8 \\
\end{align}$
