I don't understand this step marked in red colour in pic please explain briefly - Maths - Statistics

Question

I don't understand this step marked in red colour in pic please
explain briefly

Kuldeep Verma · Accepted Answer

Dear student
<img alt="" src=
"https://s3mn%20mnimgs%20com/img/shared/content_ck_images/ck_5e11bd5cf278b%20jpg">regards

I don't understand this step marked in red colour in pic ...please explain briefly