When you set

, you end up with the differentials

. Multiplying both sides by 2 gives

.
Then the integral is
We can assume that this growth rate can be expressed as y = mx + b as long as the slope is constant, because a quadratic or exponential formula wouldn't make much sense in this situation. If it is growing at least 2 inches a month, the slope (m) will be 2. As x (month) increases by 1 month, y (plant's height) increases by 2 inches. You could make it more, but as long as you plot the points in such a way that the slope for these points are 2 or higher, you will get the answer correct. If you have any other questions or need to clarify more about what you wanted let me know and I'll help.
When writing equivalent expressions, there are often several possible orders in which to simplify them. However, they will all take you to the same result as long as you do not make a mistake when using the properties. In this example, you will distribute the outer exponent first using the Power of a Product Property.