Hello! :)

I took this test a week or two ago and wanted to go over all my wrong answers in preparation for another test coming up, but this problem seems to have no explanation as to how they got the answer 1600. Now, I realize since this is a congruency problem I have to do some kind of work with the congruent triangle in order to get the value of x, but how exactly do I go about this? Please explain as much as possible- thank you! :)

-Neo