Jorge,
I do like the algebraic solution. However, there are some cases when
you can't solve your equation algebraically or when it's beyond your
abilities to solve. In those cases, I'd suggest that you use the Goal
Seek tool.

I set up a simple spreadsheet like this:

A1: X B1: 3.3
A2: i B2: 2.5
A3: y B3: 4.1
A4: s B4: 8.7

Then I added a formula in A6, which is the left-hand side of your
equation:
A6: =B1+B1*B2.

And a formula in B6, which is the right-hand side of your equation:
B6: =B3+B4

And then a difference in C6:
C6: =A6-B6

When C6 is 0, you will have a solution for your equation.

Here's how to use the Goal Seek tool:
From the menu, choose Tools - Goal Seek...
Fill in the Goal Seek pop-up as follows:
Set cell: C6
To value: 0
By changing cell: B1

Excel will iterate until it finds a solution, and replace B1 with
3.647143. If it cannot find a solution, it will tell you as much.

Jorge wrote:
X=? the values of i, Y and S are known.
X+(X*i)=Y+S