İpucu:
$$A=\frac{14-18\cos x-9\sin^2x}{3\cos x-1}=\frac{14-18\cos x-9(1-\cos^2x)}{3\cos x-1}=\frac{9\cos^2x-18\cos x+5}{3\cos x-1}$$
$$=$$
$$\frac{(3\cos x-1)^2+4}{3\cos x-1}=3\cos x-1+\frac{4}{3\cos x-1}$$ ve $$x\in\mathbb{R}\Rightarrow -1\leq \cos x\leq 1\Rightarrow -3\leq 3\cos x\leq 3\Rightarrow -4\leq 3\cos x-1\leq 2$$ bilgilerini kullanarak sonuca ulaşabilirsin.