Given that $\displaystyle 907$ and $\displaystyle 997$ are primes, use the fact that $\displaystyle 907 \equiv -997 \pmod{28}$ to calculate $\displaystyle \left( \frac{7}{997} \right) - \left( \frac{7}{907} \right) $.

I am a bit confused about the question to start with, is $\displaystyle \left( \frac{7}{997} \right) - \left( \frac{7}{907} \right) $ about fractions or quadratic residues?

How can I approach this question?