The equation of the directrix is x = -3
Given the general equation of a parabola expressed as
(y-y0)² = 4a(x-x0) ... 1
Equation of the directrix will be expressed as x = x0 - a
From the equation given;
y² = 12x ... 2
Comparing 1 and 2;
x - x0 = x
-x0 = x - x
-x0 = 0
x0 = 0
Similarly;
y-y0 = y
-y0 = y - y
-y0 = 0
y0 = 0
Also;
4a = 12
a = 12/4
a = 3
Substituting x0 =0, y0 = 0 and a = 3 into the equation of the dirrectrix above, we will have;
x = x0 - a
x = 0 - 3
x = -3
Hence the equation of the directrix is x = -3
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