How Can You Find The Radius Of A Cylinder If You Only Know The Volume?

If you only know the volume, you can't.

You need to know a couple things... The volume and the height for example.

The volume is equal to the area of the base multiplied by the height, so if you have the volume and the height you get

V (volume) = A (area) * H (height)

or

V / h = A

Once you know the area of the base, you can solve for the radius with

A = π (r)²

or √(A/π) = r

God my math teacher is so dumb he is giving us a question by giving us the volume which is 900 ml and making us find the radius and height. It is impossible to do. Plus i am only in grade 8

Dude....to guest its not impossible thats so easy and im in 7 grade

You can't trial and error is the only way.
You need to have more data