The equation of a parabola is (y−1)2=16(x+3) .

$(y-1)^2 = 16(x+3)\\ 4(4)(x-(-3)) = (y-1)^2\\ p = 4$

The focus of the parabola is 4 units on the right of the vertex.

Therefore the directrix of parabola is x = -3-4, i.e., x = -7.