Natively, Excel does not support this.
It is usually done by creating 2 cells. One is the cell that you input text, the other is the cell with the outcome.
For example:
Cell A1 has the outcome, and Cell B1 has the input. Cell A1 will prepend the text "Answer: " to whatever is typed in cell B1, but only if Cell B1 contains text. The formula in Cell A1 would be:
=IF(B1="";"";CONCATENATE("Answer: ";B1))
If you really want to just use one cell, you will be required to use VBA macro's to add the after a cell changed. Do note, that if you go this route, you will also take into account that if a user edits a cell again, the added text is now in the cell too, and upon submitting, it is added again. So the macro has to see if the text was in there to begin with, and skip adding it a second time.
Usually it easier to create a small button near that cell that pops up an input field and calculate what the outcome would be based on that input.
That said, VBA scripting is very advanced, and unless you actually can provide code of your own, is considered off-topic here on SuperUser.