Let Peter's hit be x and Alice's runs be y.
x = 2(y - 6) . . . (1)
x + y = 18 . . . (2)
Putting (1) into (2) gives,
2y - 12 + y = 18
3y = 18 + 12 = 30
y = 30/3 = 10
x = 2(10 - 6) = 2(4) = 8
Therefore, Peter hit 8 home runs and Alice hit 10 home runs.