Answer:
10²- y²
Explanation:
Since the mural is 10 feet on one side we can make the size of Ociel's mural just 10². Of course, the same would go for Bill's mural except we square however long one of his sides is. So just subtract Bill's (y²) from Ociel's (10²).
I think that's right... Sorry if I'm wrong the question was just a bit confusing.