I need help on number 7

A triangle adds up to 180 degrees, so...on triangle DCE if you add 60 and 53 you get 113 and if you add the missing side, in this case the missing side is 67 you get 180.

You can also prove this with the smaller triangle. If you add 60 and 67 you get 127 and if you add 53 all the angles add up too 180 degrees.