If you want the rate of change of the height of the child between years 5 and 2, it'll be [LaTeX ERROR: Convert failed] .

If you want the average rate of change of the height between those years, you need to divide the above expression by the number of years that have passed: [LaTeX ERROR: Convert failed]

Now, the expression you have given, [LaTeX ERROR: Convert failed] is just the amount that the rate of change has changed. That is you are measuring the difference between how fast the child is growing at 5 and at 2 years.