The tree started at 8 feet tall then grows a given amount per year.
You need to multiply the growth per year by the number of years and add it to the beginning height:
So for the Sycamore tree it would be 8 + 2.5x, so for 5 years it would be 8 +2.5(5) = 8 + 12.5 = 20.5
You do not show the possible answers, but you should be able to use the above example to figure it out.