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