I'm stuck one this problem help?

(  5xy^4 ) / ( 10xy ) = ( 5 / 10 ) * ( x / x ) * ( y^4 / y ) = ( 1 / 2 ) * 1 * ( y^3 ) = ( y ^3 ) / 2;

but, z^( - 2 ) = 1 / ( z^2 );

then, [ (  5xy^4 ) / ( 10xy ) ] ^ ( - 2) = [( y ^3 ) / 2] ^ ( - 2 ) =  [ 2 /( y ^3 )] ^ ( -2 ) =
 [( 2^2) / ( y^3)^2] = 4 / ( y^6); where x and y are not 0.