Solve equation

With
A1: = B1* LN(B1/84)
B1: a seed value to start with....100

Data.What-if-Analysis...Goal Seek
...Set cell: A1
...To value: 28.42
...by changing cell: B1
...Click: OK

Does that help?

Can you specify what error Solver is giving you?

Rather than just give you the solution, let's talk a little about how to solve these.

1) Solver's default is to use the Newton Raphson method to find the roots to these kinds of equations. If you are unfamiliar with how the NR algorithm works, I would suggest you spend some time becoming familiar with it. Wikipedia has a good discussion, even if it tends to be a very technical. I like this page from the University of Utah that has a nice little applet for exploring the behavior of the NR algorithm with several different problems.

2) One of the first things I usually do with a problem like this is create a table of values to show C as a function of x. Maybe something like (comma delimited):

B,84

x,C
1,=A4*ln(A4/$B$1)
2,copy of B4
5,copy of B4
10,copy of B4
20,copy of B4
and so on

Then plot on a scatter plot. Then I can observe the behavior of the function and graphically visualize what it is doing. Do this for several values of B to see how B effects the function.

3) A most useful part of doing part 2 is that, once I have an overall picture of how this function behaves, it becomes easier to select an initial guess for x (the tutorial above should have shown how important initial guess is to the NR algorithm). This function is fairly simple, I would expect that Solver should have no problem finding the solution once you give it a suitable initial guess.

However 28.42 is not a fixed number and is an outcome of calculations and will change for other scenarios. Can i reference the "to value" in goal seek to a cell.

You cannot directly reference a cell in the target value field in Goal Seek/Solver. The way I would do it is to add a helper cell that is simply "Set cell"-"target value". Then have Solver/Goal seek set helper cell to a value of 0 by changing x.

It says-"Solver encountered an error in the objective cell" Please help!

Gut feel is that this error is ultimately because you chose a bad initial guess (what Ron called a seed value). I will suggest again that you review the tutorial I suggested so that you understand how the NR algorithm works. For this problem, it is necessary to select an initial guess that is on the right side of the function minimum. If your initial guess is on the wrong side of the function minimum, then the NR algorithm will naturally try a negative value for x, and Excel cannot take the ln() of a negative value, and, thus you get an error. To make this spreadsheet more robust, it might even be worthwhile to spend some time with the function (plotting the function at several different values of x as I suggested would be a good start). There should be a way to have Excel calculate a good initial guess (perhaps by locating the minimum and selecting some value for x larger than that).