If the function showed linear growth, it would be a straight line. Since it curves upward, it will be either "exponential" or "other." The growth factor seems more or less constant, so we can claim the curve to be exponential.
When x=1, y=1; when x=2, y=3. The increase in x by 1 unit causes the value to grow by a factor of 3/1 = 3. When x=3, y=9, which is growth by another factor of 3. The growth factor is 3.
