Question

algebra (pic in comments)

Answer 1

Let’s let L = price of one large envelope and P = price of one postcard

Using the first line, if a customer paid $12 for 14 postcards and 5 large envelopes… then 14 postcards costing P dollars each cost 14*P or simply 14P, and using the same logic, 5 envelopes cost 5L.

Since 14 postcards and 5 envelopes cost 12 dollars then

14P + 5L = 12

Using the same logic, can you try writing an equation for the second customer and then solving the system for L?