If a value of $y$ is given and we know that $x$ and $y$ lie on a specific straight line so that $y = a + bx$, then we can find the value of $x$ by considering $y = a+bx$ as an equation to be solved for $x$, since $y$, $a$ and $b$ are all known. \\

The general solution is found through the following steps:
\begin{itemize}
\item Equation: $y = a + bx$
\item Subtract $a$ from both sides
\begin{itemize}
\item
$y-a = bx$
\item
$bx=y-a$
\end{itemize}
\item
Divide by $b$ on both sides if $b$ is not equal to 0.
\begin{itemize}
\item $x=\frac{1}{b}(y-a).$
\end{itemize}
\end{itemize}