I think what you have to do is add up A B and C so you know how much space they take out of the 456 cm which is the length of the wall. Then you subtract 456cm (which is the total) *the length of the wall* by the result you got from the addition. And then the result you get divide it by 2 so it would give you what is the width of D and E since it says they're equal.
But the equation Idk.. still got me thinking
Hope that helps.. :)