Question 1080880
Twice the difference is between the third and fifth, and that difference is x+24-(3x+8)=-2x+16.
So, the common difference is -x+8.
Between the fifth and eight is 3(-x+8)=-3x+24, and that plus x+24=x^3+15.
x^3+15=-2x+48
x^3+2x-33=0
graph it and x=3.  Common difference is -6+16=10, between 3 and 5, so half of that is 5 ANSWER.
check between 5 and 8, where there should be a difference of 15.
x^3+15-(x+24)=x^3-x-9=15
Check between 3 and 8, where there should be a difference of 25
x^3+15-(3x+8)=x^3-3x+7=25
{{{graph(300,300,-10,10,-10,10,x^3+2x-33)}}}