Hiding fields when drop down selected

Hi

I have a drop down box with 3 choices - Telephone/Writing/Not applicable

If telephone chosen I want "text8" field box to show
If Writing "test7" to show

if Not applicable chosen - I want neither box to show....

This below isn’t working  - when I select "not applicable" I still get the “text 8”
box displaying (but not the “Text7” box) (I am brand new to this) – what is wrong
please!!

Thanks

if (event.value == "Not applicable")

{

this.getField("Text8").display
= display.hidden;

this.getField("Text7").display =
display.hidden;

}

else if (event.value == "Telephone")

{

this.getField("Text8").display =
display.visible;

this.getField("Text7").display =
display.hidden;

}

else if (event.value == "Writing")

{

this.getField("Text7").display =
display.visible;

this.getField("Text8").display =
display.hidden;

}