Greg has nickels and pennies in his pocket. The number of pennies is three less than twice the number of nickels. Let n represent the number of nickels

It seems that you have missed few parts of question. So I searched for the similar question on internet which is as follows:

"Greg has nickels and pennies in his pocket. The number of pennies is seven less than twice the number of nickels. Let n represent the number of nickels. Write an expression for the number of pennies."

If the actual question has different numbers then also you can solve that problem as this problem.

Given that number of nickels = n

Now it says that "The number of pennies is seven less than twice the number of nickels" so we can write:

2n-7

Hence final answer will be (2n-7).