Answer :
8 / 4 = 2 ;
z^8 / z^4 = z^4 ;
then, 8z^4 / 4z^4 = 2z^4 ;
finally, ( 8z^4 / 4z^4 )^ -4 = ( 2z^4 )^ - 4 = 1 / ( 2z^4 )^4 = 1 / [ 2^4 * ( z^4 )^ 4 ] = 1 / ( 16 * z^16 )
z^8 / z^4 = z^4 ;
then, 8z^4 / 4z^4 = 2z^4 ;
finally, ( 8z^4 / 4z^4 )^ -4 = ( 2z^4 )^ - 4 = 1 / ( 2z^4 )^4 = 1 / [ 2^4 * ( z^4 )^ 4 ] = 1 / ( 16 * z^16 )
