I'm not sure what the question is, your reasoning is correct. Just remember that the b you found is only half the length of the diagonal.
b = sqrt(63) = sqrt(9 * 7) = 3*sqrt(7)
2*b = 6 sqrt(7)
Each side of a rhombus measures 12 in. If one diagonal is18 in. long, how long is the other diagonal?
So what I did is. Constructed a rhombus. Made each side 12 in and made a diagonal of 18. I make another diagonal and so I get 4 right angles
I choose 1. I have 12 as c^2 and 9 as a^2 now. I need to find B^2
9^2 = 81 + b^= 144
b = 63
correct answer is --- > 6√7