8^6 can be written as 2^6*2^6*2^6 or in other words 2^18.
4^7 by the same means can be written as 2^7*2^7 or 2^14.
split it into two fractions with the same denominator:
2^18/2^14 - 2^14/2^14.
The first term simplifies to 2^4 and the second term simplifies to 1.