Find which value of x....horizontal Tangent Line 1. The problem statement, all variables and given/known data What is given is F(x)= -4/(x-3)(x+4) and the problem asks for to find the value of x where f(x) has a horizontal tangent line. 2. Relevant equations I read somewhere else on these forms that using the quotient rule is the key, and that you make F'(x) to be equal to 0. If this is correct, I need some clarification on how it'd be put into the formula.