First subtract how tall it was 7 1/2 years ago from how tall it is now.
63 1/2 - 18 1/2 =
63 1/2 - 18 1/2 = 45
So in the 7 1/2 years the tree grew 45 feet. To get the average yearly growth for the 7 1/2 year period you have to divide the 45 feet by 7 1/2 years. = 6
It makes it much easier if you change the fractions to decimals.
63 1/2 = 63.5
7 1/2 = 7.5
63.5 ÷ 7.5 = 8.46(repeated)7 feet per year so 6.500 rounded
