The sequence is arithmetic since the common difference that is equal
d=11-15=7-11=-4
a(n) = a1 +(n-1)d
a1=15
=
a(n) = 15+(n-1)-4
81=81•2/2=162/2
81 1/2=162/2+1/2
=163/2
So 81 1/2=163/2=81.5
Remove the radical by raising each side
6^2= square root 8x-4 ^2
36= 8x-4
40= 8x
X=5
