If the 3rd and 9th term of an ao are 7 and -8 respectively, which term of this ap is zero
you have 6 terms in between these two, so d = -15/6 = -5/2
That means that a_(n+2) = a_n - 5
Sorry, no amount of adding -5/2 to 7 will get you to exactly zero.
I suspect a typo