Verified answer

is that -(√x) + 16 or -√(x+16) ?

remember that x^y = z is the same as log[base x](z) = y

so 6 to the power of what is 216? 6*6*6 = 216, thus y=3

log[6](216) = -√(x + 16)

3 = -√(x + 16)

-3 = √(x + 16)

(-3)² = (x + 16)

9 = x + 16

9 - 16 = x

x = -7

QED.