Answer:
a = 9
Explanation:
Since QS is an angle bisector then the following ratios are equal
=

[ note that SR = 24 - a ]
substituting values into the equation
=
( cross- multiply )
25a = 15(24 - a)
25a = 360 - 15a ( add 15a to both sides )
40a = 360 ( divide both sides by 40 )
a = 9