Firstly, Ms. Carger takes her 3 minutes to start up her computer and log-in to her email.
Therefore in x minutes, after starting up her computer and logging-in to her email, she would have x minutes minus the time used to start up her computer and log-in to her email. this implies she would have (x-3) minutes left for sending emails.
Since it takes 7 minutes to write and send each email, in (x-3) minutes she would be able to send only emails.
Therefore equation to represent the number of emails she can send in x minutes is emails