4+(6+27) identify the property you used

How do i do this activity, is appreciated

Bring the fraction a/a-4 to a denominator of 16-a^2

Aclassroom is made up of 11 boys and 14 girls. the teacher has four main classroom responsibilities that she wants to hand out to four different students (one for each of the four students). if the teacher chooses 4 of the students at random, then what is the probability that the four students chosen to complete the responsibilities will be all boys?