(6a^-4b^2)^-2 for the ^ it is an exponent,

Negative exponents work the same as positive ones, except you have to consider the inverse of the base.

So, if for example , in the same fashion you have

So, negative exponents lead to a fraction with numerator 1 and denominator the same expression, but with positive exponent.

So, in your case, let's work with the inner parenthesis first:

Now, if we want to raise this to the negative two, we have

this doesn't make sense lol