You need to understand what the exponent is applied to.

means raise 6 to the 4th power, and then take the negative of that. In this case, you apply the exponent only to 6. The negative sign comes later.

Now the other case.

means the 4th power of the number -6. Now the base is -6, and the exponent is 4. You need to do a multiplication in which you see the base, -6, appearing 4 times as a factor in a multiplication.

Since there is an even number of negative signs, the answer is positive.
