In order to solve a literal equation, we just need to isolate the chosen variable in one side of the equation. So we have:
[tex]\begin{gathered} u=x-k \\ u+k=x \\ x=u+k \\ \\ 12am=4 \\ a=\frac{4}{12m} \\ a=\frac{1}{3m} \\ \\ a-c=d-r \\ a=d-r+c \end{gathered}[/tex]