Interest Payments per Month

Hello, i'm hoping someone can help me with this, but i am trying to figure out a constant monthly payment for 336 months to pay off a loan with 6% interest - and by month 336 that the remaining balance equals to $0

it's hard to explain, but here is my excel sheet and the formula i'm trying to figure out would be under I27 in the "Buying" sheet. The cell next to it should hit $0 by month "360" (since in this problem the guy wanted to pay $300 a month for the first 2 years)