Prep-PS1-177 PS題求思路

一道PS題求思路~

請問如何列算式?

When a certain tree was first planted, it was 4 feet tall, and the height of the tree increased by a constant amount each year for the next 6 years.  At the end of the 6th year, the tree was   taller than it was at the end of the 4th year.  By how many feet did the height of the tree increase each year?

The answer is 2/3.