Clearly 9,7,5,... is an AP with first term a=9 and common difference d=−2 and 24,21,18,.... is an AP with first term A=24 and common difference D=−3.
LetanandAn be the nth term of first and second AP respectively.
Therefore according to question,
an=An
⇒a+(n−1)d=A+(n−1)D
⇒9+(n−1)×−2=24+(n−1)×−3
⇒−2n+11=−3n+27
⇒n=16