How do I insert a calculated value into a text cell?

f[x_] := x^3 + x^2 + 1

Row[{"The value of this function at ", 7 , " is ", f[7], "."}]

Or

StringTemplate["The value of this function at `1` is `2`."][7,  f[7]]

f[x_] := x^3 + x^2 + 1

1. Create a text cell containing:

   $\qquad$The value of this function at 7 is N[f[7]]

2. Select N[f[7]] in the cell

3. Choose Evaluate in Place from the Evaluation menu or that operation's keyboard shortcut (Cmd+Return on OS X)

To create an actual text cell, as opposed to an output cell containing text, use

f[x_] := x^3 + x^2 + 1 
CellPrint[{TextCell[Row[{"The value of this function at ", 7, " is ", f[7], "."}],
    {"Text"}]}]

(* The value of this function at 7 is 393. *)