Answer:
Your question is unclear so I'm just gonna take my points and go...
(X+y)(x-y) this is the answer
I think it is 150 but i am not that suere.
So for the first one what you did was wrong as in wrong formula.
Radius is 5cm as it is given and diameter is double the radius which is 10.
So r=5
And D=10.
For the second one the diameter is 16cm as it is given and radius is half the diameter,so half of 16 is 8cm right?
So r=8
And D=16