I am making a quilt and am fairly new i need help with a measurement?

my baby quilt will measure 36 by 42, i am putting my nieces name in quilt blocks and i am placing it at a diagonal. I cant seem to figure out what the diagonal measurement would be for a 36 x 42 rectangle. I am 41 years old and have had a stroke..i feel like an idiot for asking this should be common sense..please help first to answer gets the points!