(Solved): How to find the y-value and how to generate those line use Excel software? I already watch the YT bu ...

How to find the y-value and how to generate those line use Excel software? I already watch the YT but still didn’t find understand how to do so, most of them just show the dot or calculate the intercept values only..  But not the plotted graph (the 90 angle plotted, like how the dashed line shows in this image). Anyone knows how the step to generate that line? Please show the steps with picture too, if could.