I suppose that (1) and (2) are intended to be labelnumbers of equations. Enter these via Ctrl-L:

Download Labels.mw

I understand that you read Dutch. So see https://avanmeer.home.xs4all.nl/maple/2010/ , choose Module 7, for a complete explanation.

I will not do your homework, but here is a hint: f is a polynomial with 5 unknown parameters. So you have to solve 5 equations with 5 unknowns.

See for example: https://www.mapleprimes.com/posts/200902-Colorbar-For-A-3D-Plot-In-Maple-20151

If you replace

z:=H/mg*(cosh(mg/H*x+zeta)-cosh(zeta));

  by

z :=  x -> H/mg*(cosh(mg/H*x+zeta)-cosh(zeta));

the shifted plot wil be:

Why should you use a procedure if you don't want to use a parameter sequence? But if so, you can refer to parameters outside the procedure via the -: operator. A simple example:

Download NoArguments.mw

The assignment x := LinearSolve(A, b) makes x a Vector (with parameter _t2), so it cannot be used as a range parameter. Replace it by v := LinearSolve(A, b), and don't forget to restart.

R2 is an 1x1-matrix, as to expect:

lprint(R2);
Matrix(1, 1, {(1, 1) = 44}, datatype = anything, storage = rectangular, order = Fortran_order, shape = [])

Make an operator that for multiplies the identity function with the derivative:

Download D_operator.mw

Because x and y are constants, there is no reason to try to calculate the sum symbolically.

add(binomial(y, i)*(1/3)^i*(2/3)^(y-i)*(add(binomial(x, j)*(1/3)^j*(2/3)^(x-j), j = i+1 .. x)), i = 0 .. y);

Make the colorbar as a separate 3D-plot, viewed from above (P2 in the worksheet)

If you don't want to plot (a) solution curve(s), use dfieldplot:

If you use a loop to place (some) answers in a set, it is more efficient to use a sequence as intermediate:

Download Makeset.mw

To interchange the horizontal and the vertical axis, make F13 a parametrisized plot:

The x=X substitution is not necessary: