How do do you center a poster that is 21”x 42” high to an 8’x11’ wall

Answer 1

for the width, see how much too wide the wall is: 8' - 21" = 6'3"
So, you need half of that on each side: 3'1.5"
Place the left side of the picture 3'1.5" from the left wall. That will put the right side 3'1.5" from the right wall.

Follow the same procedure with the height.