(-7x-3y)(-7x-3y)(-7x+3y)(-7x+3y)
=(-7x-3y)²(-7x+3y)²
=[(-7x-3y)(-7x+3y)]²
=[(-7x)²-(3y)²]²
=(49x²-9y²)²
还要往下写吗,自己填上吧.