Based on the data given:
400 games were sold after 11 months
800 games were sold after 21 months
1500 games were sold after 36 months
We need to plot a graph of months against the number of games:
From the graph,
Yes, a single linear model is a reasonable option for the data
The equation of the graph is given by:

Note that y represents the number of months and x represents the number of board games.
The number of board games sold after 48months simply means that y= 48. In order to get x, we substitute y = 48 into the model

Therefore, the estimated number of games sold after 48 months is 2020.