Trying to solve for x

So basically I have this equation, and I'm trying to find where the second derivative of it equals 0, but I keep getting that x = 0 and I know it's not. What am I doing wrong?

syms x % Defining unknown variable to solve for
eqn = 1/(7.221*10^(-9)*x^3 - 2.353E-5*x^2 - 2.094E-3*x + 132.747); % Equation taken from polyfit
ddc = diff(diff(eqn)) == 0;
sol = solve(ddc,x);
compliance = double(sol);