the height of wall "a" is 5/2 higher than wall "b" the total height height of both walls is 420 ft what is the height of both walls.
The language is ambiguous. If the exercise had said "5/2 times as high", then b = (5/2)a; if the exercise had said "5/2 times higher than", then b = a + (5/2)a. But "5/2 higher than" reads like b = a + 5/2, which is highly unusual.
You might need to look in your book for an example that uses the same language, so you can see how the book interprets it (and thus how you should interpret it). Otherwise, you may need to ask your instructor for clarification.