Box 21 is only a code identifying the slip as either "O" original or "A" amended.  It does not need to be entered on your return.

Box 42 is a return of capital usually, which is your own money, and is not reported.

You can look at the box numbers for taxable income here.

http://www.cra-arc.gc.ca/E/pbg/tf/t5/t5-fill-14e.pdf