Answer: 4
===================================
Rule: The geometric mean of two numbers x and y is sqrt(x*y)
In this case, x = 2 and y = 8
So we have the geometric mean equal to sqrt(2*8) = sqrt(16) = 4
If you wanted to find the geometric mean of three values, then you'll multiply the numbers and take a cube root. If you had four values, then you multiply and apply a fourth root. And so on.