Two functions are equal, if they have the same domain and if the function values are the same everywhere.

in our case we have two functions f(n) = 1/(7n-4n) and g(n) = 1/3n

the domains of f and g are the same D(f) = D(g) = R\{0}

for arbitrary x from the domain R\{0}

f(x) = 1/(7x - 4x) = 1/(3x) = g(x)

thus f(x) = g(x) for every x from the domain R\{0}

the functions f(n) and g(n) are equal and 1/(7n-4n)=1/3n is identity.