A first number plus twice a second number is 5,
This would be x + 2y = 5
Twice the first number plus the second totals 19.
This would be 2x + y = 19
x - x + 2y = 5 - x
2y = 5 - x
2y/2 = 2.5 - 1/2x or y = 2.5 - 0.5x
You cant find the value of the numbers if 2 out of 3 of the numbers are missing.