if the focus and directrix of a parabola are(3,-4) and x+y+7=0 then its length of latus rectum is??

Given,
focus=3
directrix=-4
equation of parabol, y^2=4ax
put =3 and y=-4
16=4 a 3
16=12a
a=16/12
a=4/3
therefore,Length of latus rectum=4a
                                                   =4(4/3)
                                                   =16/3