Can somebody please tell me the value of x

Triangle knm is what is true about the sides of knm

Sanderson is having trouble with his assignment. his shown work is as follows: − 3 7 7 3 = − 3 7 × 7 3 = − 21 21 = −1 however, his answer does not match the answer that his teacher gives him. complete the description of sanderson's mistake. find the correct answer.

Will mark brainliest! ( explain the answer too, it can be one sentence)