What date on the cosmic calendar did the milky way galaxy appear?

Two rigid transformations are used to map δhjk to δlmn. the first is a translation of vertex h to vertex l. what is the second transformation? a reflection across the line containing hk a rotation about point h a reflection across the line containing hj a rotation about point k

Which of the following are guidelines to follow for obtaining accurate observations

Select all the correct answers. which three statements about electromagnetic radiation are true?