Find the equation of a parabola that satisfies the given condition:
Focus (0 – 3); directrix y = 3

The focus of parabola is (0, -3) which lies on y-axis. Directrix of the parabola is y - 3 = 0 which is parallel to x-axis.
∴ The equation of the parabola is of the standard form                  ...(i)
Focus is (0, -a)  (0, -3) and directrix y - a = 0 is y - 3 = 0     a = 3
Hence, form (i), the equation of the parabola is 
Alternative method:
Let l be the directrix with equation y - 3 = 0.
S (0, -3) is the focus.
Take a point  on the parabola. From P, draw PM perpendicular on the directrix l and join PS. By definition of parabola, PS = PM
           
            
            
Hence, the equation of locus of P i.e, equation of parabola is