Please step by step teach me how to do this question.

I don't understand why it have to times 50, where did it get 50?

This question is already with correct answer.

6. You short the E-mini S\&P500 futures when the futures value of the index is 3,150 . You close your position after 3 weeks when the futures price is 2,820 . What is your gain/loss? Gain/loss =(31502820)50=+16,500 (gain of 16,500 )