Creating a basic loop to solve an equation multiple times

Hello, and thanks for any input, i'm pretty much clueless when it comes to excel. I borrowed a couple hint books and searched online but it all seems rather confusing. I could do what I'm trying in a program like matlab but can't seem to grasp how to in excel.

Basically I have an equation with 4 known values. I'm looking to solve for the 5th value. What I would like to do is solve for that 5th value multiple times, then graph that value.

How would I create a loop to call on cell's within excel? I found a general way to create a loop in Visual basic, but not how to call the cell values then store the unknown value I solved for.

I created a simple spreadsheet where the user will enter the 4 known values, as well as the max amount for the y axis of the graph. I want to program it so it will run this loop in a set itteration up until the max value.

Do I need to store the 5th values someplace within the spreadsheet or can I "hide" those to the user and still graph them?

Thank you for any help, links, or guides that would point me in the correct direction.