Question: what is a difference of two numbers?
Answer: When you have a sum and a difference and add them together, that is the same as doubling one of them. For example, if the two numbers are 7 and 4, then the sum is 11 and the difference is 3. ... The sum is a+b. The difference is a-b. Adding the sum to the difference is (a+b) + (a-b) which is a + b + a - b = 2a.