First subtract how tall it was 7 1/2 years ago from how tall it is now.
57 - 40 1/2 =
57 - 40.5 = 16.5 or 16 1/2
So in the 7 1/2 years the tree grew 16 .5 or 16 1/2 feet. To get the average yearly growth for the 7 1/2 year period you have to divide the 16 1/2 feet by 7 1/2 years.
It makes it much easier if you change the fractions to decimals.
16 1/2 = 16.5
7 1/2 = 7.5
16.5 ÷ 7.5 = 2.2 or 2 1/5 feet per year
Hope this helps. :)