you can try returning an array from your function FtoC like below:

function FtoC(Fahrenheit)
{
return [Fahrenheit, (Fahrenheit - 32) * 5 / 9];
}

var theFahrenheit = FtoC(45.9);

document.write("<br>");

document.write(theFahrenheit[0] + " F = " + theFahrenheit[1] + " C");

or Else if you already know the input, you can directly use it instead of returning it in an array.

document.write(45.9 + " F = " + theFahrenheit[1] + " C");

Try this:

<script>
  function FtoC(Fahrenheit) {
    return (Fahrenheit - 32) * 5 / 9;
  }

  let temperature = 45.9;
  var theFahrenheit = FtoC(temperature);
  document.write("<br>");
  document.write(temperature + " F = " + theFahrenheit + " C");
</script>

On this line:

document.write(FtoC + " F = " + theFahrenheit + " C");

You're writing out FtoC, which is your function, not the temperature. Try introducing a new variable to hold the initial temperature, and write that out to the document instead.

<script>
    function FtoC(Fahrenheit)
    {
        return (Fahrenheit - 32) * 5 / 9;
    }

    var theTemp = 45.9
    var theFahrenheit = FtoC(theTemp);
    document.write("<br>");
    document.write(theTemp + " F = " + theFahrenheit + " C");
</script>

I also noticed in that line that you're writing theFahrenheit + " C" which seems backwards. Since the variable theFahrenheit is holding the temperature after conversion, it should be named something else.