Take logs of both sides: (x-2)log8=log(1/16)=-log16. If we go for log to base 2 we get 3(x-2)=-4. So 3x-6=-4 and 3x=2, making x=2/3 or 0.667.
If the question was (8^x)-2=1/16, 8^x=33/16. Logs to base 2 of both sides: xlog8=log(33/16)=1.04439. So x=1.04439/3=0.3481 approx.