The sum can be rewritten as y=4x, where y=f(x).
Now, we can rewrite the equation a x=y/4
Therefore, inv(f(x))=x/4
Hello.
In 10 years, his age will be

The sum of his current age and 10 equals his age in 10 years.
2 years ago his age was

The difference of his current age and 2 equals his age 2 years ago.
7 years ago his age was

The difference of his current age and 7 equals his age 7 years ago.
Therefore, his age after 10 years will be
, his age 2 years ago was
, and his age 7 years ago was 
I hope it helps.
Have a nice day.

3/8 times 8/3 = 1
Answer=8/3
Has to be greater than 90 degrees.