Marina bought 4 notebooks that cost b dollars each, and 3 pens that cost c dollars each. How much money did Marina spend?

4b+3p. there should be a value for b and c.

4B+3P. This is because B represents how much each notebook costs, and if we multiply it by four (because Marina bought four) than that's how much the notebooks cost. An the same works for the pens, an we need to add them together so we get 4B+3P.