( f / g )(x) = f(x) ÷ g(x)
( f / g )(x) = ( x^2 - 7 ) ÷ ( - 6x + 5 )
( f / g )(x) = x^2 - 7 / - 6x + 5
( f / g )( 5/6 ) = (5/6)^2 - 7 / - 6(5/6) + 5
( f / g )( 5/6 ) = 25/36 - 7 / - 5 + 5
( f / g )( 5/6 ) = ..... / 0
A zero appeared in the denominator, so the given number is the root of the denominator thus it is not in the domain of the function that wants to have a value.