This problem OG seems to be causing much trouble but it is really not that complicated. I already solved this in the past but will try to be more precise in my explanation this time. In the below diagram, you will see that there are 4 variables you need to find {x1,x2,y1,y2}. Remember that right angle has to be at P. P will then have the same x-value as Q (x1 it is) and R will have the same y-value as P (y1 it is). Now, this is where this will become a permutation problem.

Let's start with the x's:

You have 10 ways of picking x1 for P(from -4 to 5 inclusive)

You have 9 remaining ways of picking x2 for R

Total permutations for x's: 10*9 ways of picking 2 values where order counts

For y's:

You have 11 ways of picking y1 for P(from 6 to 16 inclusive)

You have 10 remaining ways of picking y2 for Q

Total permutations for y's: 11*10

Total permutations, number of ways of picking 4 values, x1, x2, y1, y2, to form a right triangle: 11*10*10*9 = 9900

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Best Regards,

Paul