7th term = (6th term)*r
8th term = (7th term)*r

a7 = a6*r
a8 = a7*r

a7 = 8*r
8 = a7*r

Substitute 8*r for a7 in the second equation:

8 = (8*r)*r
8 = 8r²
Divide through by 8
 1 = r²
r² = 1
 r = ±1

So there are two solutions:

a7 = 8*(±1)

So a7 is either 8 or -8.

Maybe your teacher only wanted the positive solution.
You'll have to ask.

Or maybe your teacher wanted you to just use the formula
for the geometric mean:



You'll have to ask your teacher.

Edwin