The problem says: "Bob has three times as many baseball cards as Bill." Mathematically, this means: X = 3*Y.
The problem also says: "Together they have a total of 68 cards". Again, mathematically, this means: X + Y = 68.
We now have a system of equations that can be solved by the substitution method. Let us replace "x" with "3y" from the first equation into the second equation.
3*Y + Y = 68
4*Y = 68
Y = 68/4 = 17.
One can now find X by either using the first or the second equations. The result will be the same. For example, the first one gives: X = 3*Y = 3*17 = 51.
Thus Bob has 51 cards, whereas Bill has 17 cards.