Two numbers have a sum of 12 and a difference of 4. What is the smaller of the two numbers?

Which of the following is the result of expanding

Which one ? is the answer and how to find this answer

Julie goes to the sports store and spends $40.50 before tax. she buys a pair of shorts for $21.75 and 3 pairs of socks that each cost the same amount. how much does each pair of socks cost? $$