Plotting the graph with given data we have,

We know that, in any parabola, the length of the latus rectum =4a and perpendicular distance from vertex to the directrix =a
So, a=∣42+(−3)28+3−21∣
⇒a=∣5−10∣=2
Therefore, the length of the latus rectum 4a=4×2=8