A function  can only return a value or values. It can't change any cell
directly, just the one(s) that is is returning value(s) to. You could make your
function an array function, so that it can occupy multiple cells. For example:

Public Function boomLength(radius As Double, height As Double)
Dim vData(1 To 1, 1 To 2) As Variant
vData(1, 1) = (radius ^ 2 + height ^ 2) ^ 0.5
vData(1, 2) = "insert text  here"
boomLength = vData
End Function

Then, if entered as a simple function, it returns only the boomLength value.
But if array-entered (i.e. Ctrl-Shift-Enter) as a row and two columns, you'll
get both the boomLength value in the first column and the text in the second
column.