WHY TF IS THE ANSWER NOT 9 ??

https://preview.redd.it/af93a2yt8goe1.png?width=558&format=png&auto=webp&s=2da25aa3c81bea3a76a1bf52d1ba29078b9cb229

c=cv +r/1-j then to calculate j i assumed the process to be a random pv ^j then differentiated it to get the slope -jp/v then equated it with the slope of the given graph which is po(n-3)/4vo from this j comes out to be =3-n/12 on putting it in the eq i get n =9 what am i doing wrong here pls someone explain