Hey there!
The answer is 72 - 2n.
"twice a number" represents a number multiplied by 2. So we have "2n"
"decreased by twice a number" would mean subtracting twice a number, or subtracting "2n".
"72 decreased by" means that we are subtracting the number FROM 72, so we get "72 - 2n".
Have a super awesome day! :)