Martha made a mistake. in which step did martha first make a mistake, and what is a correct expression for that step?

We have  that
 f²{k^6/[k^4*k^3]}^-3

step 1
 f²{k^6/[k^(4+3)]}^-3> f²{k^6/[k^7]}^-3> step 1 is correct

step 2
 f²{k^6/[k^7]}^-3>  f²{k^(6-7)]}^-3>  f²{k^(-1)}^-3> step 2 is correct

step 3
f²{k^(-1)}^-3> f²{k^[(-1)*(-3)]}> f²{k^(3)}
therefore
Martha made a mistake in the step 3

the answer is
step 3
f²k³