Given data:
$39 for unlimited rides
$12 for admission plus $1 per ride
Let x be the number of rides
Let y be the cost in terms of the number of rides
First option is always $39 (it doesn't depend of the number of rides)

Second option is $12 plus $1 multiplied by the number of rides:

System of equations:

The two options wind up costing the same amount when the equations are equal:
y=y
Subtitute the value of y corresponding in each equation:

Solve x:

Then, the two options wind up costing the same amount at $39 and it correspond to 27 rides