inverse of the matrix will be as follows:
1/a -f/ab (fd-be)/abc
0 1/b 0
0 0 1/c
Since a,b,c,d,e and f are real numbers so, all the numbers in the above matrix is also real and since, a*b*c != 0, means a!=0 and b!=0 and c!=0 So, 1/a=!0 , 1/b !=0, 1/c!=0 as well , So (1/a)*(1/b)*(1/c)!=0 . Hence , inverse matrix also belongs to set H