The monthly payment is calulated by dividing the total cost of the mountain bike by 3. For example, if the total cost is $200, the three monthly payments will be $66.67 each:
8x+12=p so you would take away 12 from p making the equation 8x=p-12 and then divide that by 8 so the equation is x= p-12/8 ( x equals p minus 12 over 8)