(1)a×b-4×x×x,
=ab-4x 2,
(2)4x 2=ab-4x 2,
把a=8,b=9代入4x 2=ab-4x 2,
得,8x 2=ab,
8x 2=8×9,
x 2=9,
所以,x=3,
故答案为:ab-4x 2;3.