Question 354548
Sure you do. Let's walk it step by step
{{{E/R+r = I}}}
First thing, you need to make sure the problem is written clearly
Do you mean 
{{{(E/R)+r = I}}}
or 
{{{E/(R+r) = I}}}
As written, it is 
{{{(E/R)+r = I}}}
so let's solve that one
You want to isolate R, so 
{{{(E/R)+r = I}}} subtract r from each side
{{{(E/R) = I - r}}} now multiply both sides by R (assumes R is not zero)
{{{E = (I-r) * R}}} now divide both sides by I-r (assumes I-r is not zero)
{{{E/(I-r) = R}}}
there you have it

If the problem was actually
{{{E/(R+r) = I}}}
Then do a similar set of steps
{{{E/(R+r) = I}}} multiply bith sides by (R+r)
{{{E = I * (R+r)}}}
{{{E = IR + Ir}}} subrtract Ir from both sides
{{{E - Ir = IR}}} divide both sides by I
{{{(E-Ir)/I = R}}}

Be careful when posting problems to this site. use as many parens as you need to make the problem clear. Else folks like me can't help much