One less limb than yesterday! so let’s assume like a regular human the mutant has 4 limbs. he grows another 5 and now he’s got 9 limbs. he loses 12 so 9-12 equals -3, plus 2 is -1.
If we rotate triangle PMO so that the 6 cm side is on the bottom, it would look more similar to the triangle JGH, and make it clear that side PM corresponds to side JG