3. Let $x \in\left(-\frac{3 \pi}{4}, \frac{\pi}{4}\right)$ and $\cos \left(\frac{\pi}{4}-x\right)=-\frac{3}{5}$, then the value of $\cos 2 x$ should be
A. $-\frac{7}{25}$
B. $-\frac{24}{25}$
C. $\frac{24}{25}$
D. $\frac{7}{25}$
Solve:
$\frac{\pi}{4}-x \in(0, \pi) \Rightarrow \sin \left(\frac{\pi}{4}-x\right)=\frac{4}{5}$, thus $\cos x=\cos \left[\frac{\pi}{4}-\left(\frac{\pi}{4}-x\right)\right]=\frac{\sqrt{2}}{10}$ $\Rightarrow \cos 2 x=2 \cos ^{2} x-1=-\frac{24}{25}$, so the answer is $B$.