原式=(2a)2-(3)2=2a-3.
故填:2a-3.
The building ______ stone and each stone ______ more than two tons (吨). [ ]
A. is made of; weighed
B. is made of; weights
C. is made of; weighs
D. is made from; weighs
按照《文物保护法》的规定,受国家法律保护的文物有几类