The perpendicular distance from centre to the chord its bisects the chord.
Given equation of circle is x2+y2+2x−4y−11=0 ∴ Centre is, (−1,2).
Since, the diameter of the circle is perpendicular to the chord of the circle. Hence, the equation of diameter is x+2y+λ=0…..(i)
Its passes through the centre (−1,2) of the circle ⇒−1+4+λ=0 ⇒λ=−3
Putting the value of λ in Eq. (i), we get x+2y−3=0