If H1(x) is the inverse of H(x) , what is the value of H1 (H(x)) ?

In a function, you feed in x and get y.
In the inverse function, if you feed in y, you get x

So basically if you take the inverse of the original function, you get back the original input.

Hence:
H^-1(H(x)) = x

Take this example:
H(x) = 3x
H^-1(x) = x/3

H^-1(H(7))
--> H(7) = 3(7) = 21
--> H^-1(21) = 21/3 = 7