Answer :
[tex]\frac{c}{3}-6=12\ \ \ \ \ | add\ 6\\\\
\frac{c}{3}=18\ \ \ \ | multiply\ by\ 3\\\\
c=54\\\\Solution\ is\ c=54.[/tex]
#1 - Add 6 to each side. ... #2 - Multiply each side by 3. ... The answer jumps out at you !