let the circle be of centre(a,3)

x axis is the chord of the circle(y=0)

now when a perpendicular is dropped frm centre on to the chord of the circle it bisects the chord.

the distance frm centre to the line y=0 is 3 and the length of the half chord is 4 units.

which implies the radius is 5 units since the radius becomes hypotenuse to the triangle formed by the chord,perpendicular drawn frm centre to chord

the distance frm (0,3) to (a,3) is 5 units this mplies the value of a can be +,-5

using (x-a)²+(y-3)²=5 and substituing a=+,-5

we'll get the reqired equation