Problem with symbolic while solving an equation

Hello everyone.

I'm having a problem solving a tiny equation with units and hope you can help me find where I go wrong.

As you can see on the picture I get the right answer if I isolate Alpha myself, so I'm guessing that its how I do things that is the problem.

Sorry its in danish, but its says "Angle acceleration under breaking of a flywheel and axel"

The values on the top are defined further up in the document.

"I" is inertia.