Jenna bought n notebooks which cost 5 dollars each, and 3 pens which cost r dollars each. how much money did she spend?

You can represent the amount of money that Jenna spent by writing an expression based on the information given.

5n (this is the amount she spent on notebooks) + (and) 3r (this is the amount she spent on pens)

5n + 3r would show how much money she spent.  If you know how many notebooks she bought and how much each pen costs, you would multiply these numbers by the 5 and the 3 and add them.