@Preben Alsholm 

Thanks for the reply. 

It was a great help.

However, physically, I expected the solution to decrease with z.

Maybe the problem is in the physics behind the problem.

Thanks anyway :)

@Ramakrishnan 

Thanks for your great solution.

However, it does not always work.

Now I have to do the same thing for a more complicated equation:

(19.42795980*(1+z))/x(z)-(14.09324088*(1+z))*(eval(diff(x(z), z), z = 0))/(x(z)*x(0))-19.42795980*(1.+z)^(3/2)+14.09324088*(diff(x(z), z))/(x(z)*(1/(1+z)^(3/2))^(5/3))-19.42795980-19.42795980*z+19.42795980*sqrt(.314*(1+z)^3+.686)=0

Do you know how to solve this one? 

@Kitonum 

oh nice!!!

that is exactly what I needed!

thanks :)

@Christopher2222 

Thanks for your answer.

but the dualaxisplot option does not actually plot the function in a box.

What I really need is a box with 4 sides, all of the having tickmarks and tickmark labels.

@tomleslie

Thanks for the nice complete reply. But unfortunately, I meant something else.

I attached a screenshot and showed shere I exactly want it. 

Actually, the reason I want it is that I have an "animate(plot,[])" and I want to save it for different y amounts. But ofcourse I cannot use the plotsetup option and put the size there, cause there I cannot say which y I exactly want the animate to be saved. Or can I?

@Christopher2222 

no 

actually what I mean is the two ruler shapes on the sides of the worksheet.

I remember there was such option in the older versions

Do you get what I mean? you can find such thing in microsoft word too! there you can put two rulers on the sides of the page you are typing...

@Adri van der Meer 

WOW!!!

This looks great!!!!

Thank you so much!

I did not know that there exists an option like curve=add(...) in Curvefit.

Thanks again :)

@Kitonum 

Here is the data in maple:

data_in_maple.mw

@Adri van der Meer 

LeastSquares doesn't help at all :(

This is the image I get:

@Carl Love 

Actually, attached you can find the data I am trying to fit a function with.

Column A consists the x values and column B has the y values.

data.xls

It looks like the figure above, but I cannot fit a function with it which goes this smooth to zero.

@Kitonum 

Thanks for the help.

The options in the Curvefitting package seem so confusing to me.

My original list for both x and y are not this easy and sort, so for instance, after a long time, the "PolynomialInterpolation" does not work for it, and actually I need one finction so I really don't want to use the "Spline" option.

Could you suggest any other way-method to get to a function?

Thanks :)