You're overdoing things!

You need to practice with factors,

then quadratics.

Be very accustomed to those.

Then you have Pythagoras' theorem to master.

Firstly, zero multiplied by anything is zero...

3(0)=0+0+0

5(0)=0+0+0+0+0

and so on...

means or

So, from that, what possible values can we have for the radius ?