Answer:
<h2>"30 feet per year".</h2>
Step-by-step explanation:
The given equation is

This expression can be described as "the three grew 30 feet per year", and has grown 1,500 feet until now.
So, according to this expression, the unit rate of growth is 30 feet per year, because that's what expresses the coefficient of the independent variable.
Also, we could find the number of years took to the tree to be 1,500 feet tall

It took 50 years to be that tall.
Therefore, the unit rate in the equation is "30 feet per year".