Lets x be the number of small greeting cards and y the number of large greeting cards, the cost would be the sum of the quantity by the price of each type of cards.
Then, we can write:

Then, if we get 3 small cards and 5 large cards we can calculate the cost as:

Answer:
a) The cost of 3 small cards and 5 large cards is $23.35.
b) We can write the cost as C = 1.70*x + 3.65*y