Let us denote the number of cards that Bob has with a symbol X.

Similarly, let us call the number of cards that Bill has with
Y.

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.

That is,
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.