4,2*7,3²-4.2*2.7² / 2.1*6.4²-2.1*3.6²=

4,2(7,3²-2.7²) / 2,1(6.4²-3.6²) =

4,2(7,3-2.7)(7,3+2.7) / 2,1(6.4-3.6)(6.4+3.6)=

2*4,6*10 / 2,8*10 = 4,6/1,4=46/14=23/7=3.2/7