The linear equation would be N=2+3d. Plug in 6 for d and you get N=2+18 and then N=20. According to the linear model, there would be 20 box tops.
The exponential equation would be N=2^d. Plug in 6 for d and you get N=2^6 and then N=64. According to the exponential model, there would be 64 box tops.