6x-4x=4.2

2x=4.2

x=2.1

6*2.1=12.6

4*2.1=8.4

(12.6+8.4):2=21:2=10.5

а /b=6/4 => a=6b/4

a-b=4.2

6b/4-b=4.2

6b-4b=16.8

b=8.4

a=8.4+4.2=12.6

cр линия=(а+b)1/2*=(12.6+8.4)*1/2*=21*1/2=10.5