The reflected point of (2,1) is (−2,1).

Equation of reflected ray
y−1=5+23−1(x+2)
⇒y−1=72(x+2)
⇒7y−7=2x+4
⇒2x−7y+11=0
Let the equation of other directrix is
2x−7y+λ=0
Distance of directrix from focus
ea−ae=538, where e is the eccentricity
⇒3a−3a=538 or a=533
Distance from other focus ea+ae
=3a+3a=310a=310×533=5310
Distance between two directrix =e2a
=2×3×533=5318
So, ∣53λ−11∣=5318
⇒λ−11=18 or −18
⇒2x−7y−7=0 or 2x−7y+29=0