Ali is 12 year 9 months old and his sister aliah is 3 year 11 months old ..what is the difference in their ages ? ? how much younger alia is from ali ?

12 yr  9 months = 11 yrs and 21 months...

11 years   21 months
3 years     11 months
subtract
8 years 10 months <=== aliah is 8 years 10 months younger then ali

First step is to get rid of the square root. remember that a square root is the same as raising to the (1/2) power. see below: multiply exponents per exponent rules to get: remember you subtract exponents from the denominator and add exponents in the numerator so: simplify further: so a is the correct choice. if you don't understand this google exponent rules and it will you understand. look at the image results.