An exponential model should be used to represent the nth pattern. The squares do not have a constant growth pattern, but rather they are growing exponentially, because the pattern they are growing at is 2,4,8. A constant pattern would be 2,4,6. I would show the work on a linear graph - plotting the lines on the graph at 1,2, 2,4 and 3,8 your arrow would be curved and pointed up.
