Set up a system of equations:
x + y = 0.6
x - y = 7.2
You can solve using elimination. Add the two equations together:
2x + 0y = 7.8
2x = 7.8
x = 3.9
Then plug the number back into either of the two original equations to solve for y:
3.9 + y = 0.6
y = -3.3
The two numbers are 3.9 and -3.3