I need help with number 9

Me. i can't figure out this question. give an explanation too. .

Parabolas y=−2x^2 and y=2x^2 +k intersect at points a and b that are in the third and the fourth quadrants respectively. find k if length of the segment ab is 5.

Me i'm timed right now! a. (0,-5)b. (0,-3)c. (0,3) d. (0,5)