4M²-9=(2M+3)(2M-3)
最简公分母是(2M+3)(2M-3)
2MN/(4M²-9)=2MN/(2M+3)(2M-3)
(2M-3)/(2M+3)=(2M-3)(2M-3)/(2M+3)(2M-3)=(2M-3)²/(2M+3)(2M-3)