Newbie question on translating formulas

I’m trying to accomplish a simple notebook with several formulas f.ex 3 phase transformers, and a list of variables to use in the formulas.
I have tried solve with no success. I was hoping there was a function which would output i=u/r with the corresponding variables is this possible at all??

Of course the above example is very simple…
Any hints would be greatly appreciated..

Any chance you could pick a good example problem from your textbook that is doing exactly the kind of thing you want to do, flop the book on your scanner, crop off everything in the scan except the example, post that image here along with a brief statement of what inputs you want to provide and what outputs you want to get? With that I think someone should be able to show you how to turn that into a simple notebook. From that start you might be able to try the next more complicated example on your own.