Check the picture below.
so the wall is 146 inches wide, half of that is 73 inches.
the painting is 30 and 1/2 inches wide, half that is 15 and 1/4 inches.
if we subtract half the width of the painting from half the wall's width, the center of the painting will match the center of the wall.
[tex]73~~ - ~~15\frac{1}{4}\implies 73~~ - ~~\cfrac{61}{4}\implies \cfrac{292~~ - ~~61}{4}\implies \cfrac{231}{4}\implies 57\frac{3}{4}[/tex]
