2nd time I’m asking! I don’t understand this problem if you can help I would be great full if you could help!

James adds two numbers a + b. sally adds the same two numbers but reverse the order b + a what property of addition assures us that james and sally will get the same sum

Bob is putting books on a shelf and takes a break after filling 18 1/2 inches on the book shelf. the book shelf is now 2/3 full. how long is the entire bookshelf?

Solve the triangle. a = 21°, c = 105°, c = 5