We want to rewrite the following equation
[tex]9d+n=5r[/tex]With d as a function of the other terms. Let's start by subtracting n from both sides.
[tex]\begin{gathered} 9d+n-n=5r-n \\ 9d=5r-n \end{gathered}[/tex]Now, let's divide both sides by 9.
[tex]\begin{gathered} \frac{9d}{9}=\frac{5r-n}{9} \\ \\ d=\frac{5r-n}{9} \end{gathered}[/tex]And this is our final answer.