设s1为三角形原面积,s2为三角形后面积,高为h,
s1=1/3*6h,s2=1/3*【6+2】h,
s1+4.2=s2
1/3*6h+4.2=1/3*【6+2】h
h=2.1m
s1=1/3*6*2.1=4.2