X=6 is the answer to your question
If they have the same charge
x+x+x+x+x+x+x=-42
7x=-42
x=-6
so to check our answer
-6-6-6-6-6-6-6=-42
-12-12-12-6=-42
-24-18=-42
-42=-42
Okay well, if you make a graph, Point A (7,0) and Point B (4,6) are in the top right quadrant. and Point C (1,1) and Point D (5,3) are in the top left, top right, and bottom left quadrant (I'll link a photo of the graph.)
so the lines intersect and make it perpendicular
How ever many times it goes into 100