so next you need a tbale for bookings / hirings whatever, referring to the customers and tools tables as foreign keys
you probablyu need a rate table.. ie equipment X is charged at rate Y, or equipment x,y & z use rate 1, a,B & E rate 3 etc....
you probably need a table to hold invoices
you probably need a sub table to hold invoice details
you probably need a financial transaction table to identify what type of financial transaction is being made on the account
you probably need to table to store details of payments made, credit given
depending on how on how complex your school work assignmrnt is you may also need a discount rules table to handle discounts for a specific item.
incidentally I wouldn't call your other table 'tools', after all you never know what the entity may end yup doing, it could be sellign products \and services, it may be hiring more than just tools.... eg scaffolding, staging etc....
I'd rather be riding on the Tiger 800 or the Norton