We put in a parentheses around "the sum of a number n and seven" 
and a bracket around 3 times (the sum of a number and 7)":

First we express what's in the parentheses:

"the sum of a number n and seven" is expressed as n+7.

So now we have:

Now we express what's in the bracket:

"3 times (n+7)" is expressed as 3(n+7)

So now we have:

To express six less than something we subtract 6 from it:

That is the expression, not simplified.

Edwin