L= total wall length
w= D & E bookcase widths in inches
L= (A + B + C) - (D + E)
plug in known information
500= (200 + 75 + 125) - (w + w)
add inside parentheses
500= 400 + 2w
subtract 400 from both sides (inverse operation)
100= 2w
divide both sides by 2
50= w
ANSWER: Bookcases D and E will be 50 inches each.
Hops this helps! :)