Let's start by subtracting 3 from both sides of the equation:
[tex] -72=-3(3n+6) [/tex]
Then, let's divide by -3:
[tex] 24=3n+6 [/tex]
We can then subtract 6 from both sides:
[tex] 18=3n [/tex]
And then finally we can divide by 3:
[tex] n=6 [/tex]
