first, we can realise that if the total cost is $10 so Martin and Isabelle pays $5 dollars for each game. however, they are telling us that Isabella pays $3 more dollars at the beginning for the shoes. so let’s have some examples:
1) if they only play 1 game:
Payments:
Martin: $5
Isabelle: $8
2) if they play 2 games:
Payments:
Martin: $10
Isabelle: $13
3) if they play 3 games:
Payments:
Martin: $15
Isabelle: $18
So if you can se the relation between the payments are always the same. that means that the inclination or the slope will be the same for bout of them, however, the line of Isabelle will be 3 units upper from the Martin's line.