I am totally lost on the principles used

How do I transpose the formula to make
e=i/d² to make d the subject of the formula

When it comes to algebra I do not have a clue

See my response to your other question here: https://www.askmehelpdesk.com/mathematics/answers-transposing-mathematical-formulae-708515.html and try to use the same gebneral technique on this problem. What do you get?