The equation of a parabola is given by x2−8x−4y+32=0. Then
x2−8x−4y+32=0
(x−4)2−4y+16=0
(x−4)2=4(y−4)
Comparing with x2=4ay
Axis is x−4=0⇒x=4
a=1
Vertex is (4,4)
y=−a is equation of directrix
⇒y−4=−1⇒y=3 is equation of directrix.