For the binary op

From the definition of multiplication modulo

a*b = (m is the base of the modulo)

Here the base is 5 and the set is S = {1,2,3,4}

So, we can make a composition table as :-

The identity element is 1.

The inverse element of 4 is 4 itself.

∴ (3 4^-1)^-1 = (3 1) ^-1 = 3^-1 = 2.