Let x be the number of cards Jake and Joshua sell within one hour. Therefore, their earnings are given by the following expressions,

Then, set Ja=Jo (both earn the same amount),

Solving for x,

Then, they will earn the same within one hour only if both sell zero cards within the hour.
Graphing the system of equations,
As one can see, the intersection point is (0,6), which stands for 0 cards and $6