You can put this solution on YOUR website! If I understood the wording correctly, r is the diameter, which is divided by 2 when the value of r is found.
I think the equation you seek is: (leg)^2 + (leg)^2 = (hypotenuse)^2, which becomes (r)^2 + (r + 16)^2 = (24)^2
When you find r, divide it by 2 to find your radius.
If this is not correct, right back with more information.