Kathy takes her cat to a veterinarian every year for a check-up. last year, the difference in the cat's weight from teh year before was -1.56 pounds. this year, the difference in its weight form last year is 0.73 pounds. what is the difference in the cat's weight from 2 years ago?

To find the difference in weights from two years ago, you will need to combine the 2 differences from the two years.  

-1.56 and 0.73 becomes -1.56 + 0.73.

To add a positive and a negative number you will subtract the absolute values of the numbers AND use the sign of the number with the bigger absolute value (negative) in your answer.

|-1.56| - |0.73|
1.56 - 0.73 = 0.83
-0.83

The cat lost 0.83 (-0.83) pounds over 2 years.

You need to do x2-x1,y2-y1 and divide it by 2 and you should get (1,-3)