I tryed and tryed but nothing?

x + y = 72
x - y = 48
----------- ADD
2x ... = 120
x = 60
y = 12

Another way:
x + y = 72
y = 72 - x Substitute this for y in the second equation.
x - (72 - x) = 48
x - 72 + x = 48
2x = 120
x = 60
y = 12

72-48=12
12=12
The answer is 12