On the right side, the numbers get larger by adding three each time. On the left side, the numbers get larger by adding 4 each time. It would be the same for any other pair of numbers when you add 4 to the left, and 3 to the right. For instance, the next pair would be (25 , 21). 25 would be the X value, and 21 would be the Y value.
Subtract 19 from 39
Divide by 4