solve for r e/r=i?

Answer 1

e/r=i means
that e=ri
so r=e/i

Answer 2

Remember, that everything you do do one side of an equation you must do to the other. Here we go.

e/r = i

(e x r)/r = i x r

e = i x r

e/i = (i x r)/i

e/i = r