Writing the slide page cell by cell and then adjusting the margins is one method for doing this and it might be the simpler method. Still, I would like to present a different approach that I think will have more versatility. It essentially formats the page as a column of lines. It does use my Presentations Application.

Here pagelet is essentially a Column without the brackets and phrase is a Row without the brackets. The routine integrate formats an integral without attempting to evaluate it. (It comes from the Student's Integral portion of Presentations that allows integrals to be manipulated before evaluation). And the prime symbol used is a real character so x' is a real Symbol. A BaseStyle can be specified for the entire page and special Style constructs can be used at various places when needed. The spacing between lines is much easier to specify. There is only one cell to close up. This, of course, looks simpler in the notebook without the notation for special characters.

<< Presentations`

pagelet[
  Framed[Style[
    "The equation for the Green's function in one-group theory",
    FontFamily -> "Helvetica", FontWeight -> "Heavy"],
   Background -> Blend[{White, Brown}, 0.2], FrameStyle -> Gray],

  phrase["For a one-dimensional reactor of dimension ", 2 a, ":"],
  phrase[Spacer[60], HoldForm[\!\(
\*SubscriptBox[\(\[DifferenceDelta]\), \(x\)]\(G[x, 
        x?, \[Omega]]\)\) + (B^2)[w] G[x, x?, \[Omega]] == \[Delta][
      x - x?]/Subscript[D, 0]]],
  "with",
  phrase[Spacer[60], G[x, x?, \[Omega]] == 0, " for ", 
   "x, x? \[Equal] ", \[PlusMinus]a],
  "Then the solution for the neutron noise is given by",
  phrase[Spacer[60], \[Delta] \[Phi][x, \[Omega]] == 
    integrate[G[x, x?, \[Omega]]\[CenterDot]S[x?, \[Omega]], x?]],

  Spacings -> {Automatic, {1, 1, 1, 0, 0, 1, 1}},
  BaseStyle -> {FontSize -> 18, FontFamily -> "Times", 
    FontWeight -> Medium, 
    FractionBoxOptions -> {AllowScriptLevelChange -> False}}
  ] // TraditionalForm

The portion of the page that fits inside the regular margins of the cell is:

Thanks a lot, this works perfectly. Actually now that I learned the concept of CellMargins, I can edit my paragraph much faster by opening the cell in question by Shift-Command-E and edit the CellMargins data there manually. Thank you!

1) Select the Cell in question.

2) Open the Option Inspector with its scope set to "Selection"

3) Search for CellMargins using the search box.

4) Adjust the values in the CellMargins option to change the spacings on any one (or more) of the four sides of the selected Cell. The items in the list are of the form {{left,right},{bottom,top}}

The CellMargins documentation, http://reference.wolfram.com/language/ref/CellMargins.html, gives you detailed information on this Option.