Task 144 Explanation

The explanation says, "But this means that a=inv(A) - great! Of course if a happens to be negative we should "wrap" it around the lower bound of the field, i.e. subtract it from M.". That looks incorrect to me e.g. -10 mod 3 = 2, whereas 3 - -10 (= 13) mod 3 = 1. M should repeatedly be added to a until
the sum becomes positive.

n = bq + r => n mod q = r q - n = bq + (q - r) => q - n mod q = q - r

I wasted a bit of time on the task using the subtraction so, I think, that the text needs fixing.

Thanks for looking into this.