When you add 5x+3y to x-3y and -3 to -15, how do you know that the resulting sums are equal?

Jacks bank statement shows an account balance of -$25.50. which describes the magnitude, of size, of the debt in dollars

How do you solve a question like this?

What set does not contain -3 the set of all real numbers the set of all integers the set of all whole numbers the set of all rational numbers