In one baseball season, Peter hit twice the difference of the number of home runs Alice hit and 6. Altogether they hit 18 home runs. How many home runs did each player hit that season?

From the above information, we get 2 equations:

We take Peter's score as x and Alice's as y ...

So, X = 2 (Y-6) = > X = 2Y - 12

=> X-2Y = - 12 (first eq)

And second will be,

X + Y = 18

Solve these for x and y.