HELP: Fillable Form IF/THEN Javascript

Hi, 

I'm making an invoice form with Acrobat, it's my first time to do it.

I have no idea how to achieve what I want.

See the below image, sometimes we only have one product to write on.

I don't want the other lines to show anything.

However, I set "TOTAL2" Calculation as QTY2*UNIT2,

though no.2 has no item, TOTAL2 still shows as 0.00.

1.

I want it can show the correct multiplication when I have to enter row 2(shows the value of QTY2*UNIT2); if row 2 is blank, I hope TOTAL2 is blank, too.

**Update**

OMG with JR_Boulay help, I tried to combine this and that script(know nothing about the script) and I found a way that works!!! Not sure if it is a correct way, but it meets my demand.

var t = this.getField("QTY2").value*this.getField("UNIT2").value;
event.value = t;
if (event.value == 0) {event.value = ""};

This can make the TOTAL2 column remain blank if there's no data of item 2;

also, it'll do the multiplication script when item2 exits.

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2. Similar problem with the small square "SYMBOL", at the left side of "UNIT2".

Because we use a different currency, so I should make it changeable.

I copy&paste it, so it will show the same result of the upper currency symbol.

However, if row 2 is blank, it still shows. 

So I delete the copy one, make a new text field, but have no idea how to deal with it.

What I need is, the SYMBOL column remain blank if there's no item2;

if I have to enter item2's data, I want the SYMBOL column to show the currency symbol, which there's already a CS column in the form, I entered it manually. I hope the SYMBOL column can sync with the CS column.

**Update**

I found a way to solve it:

var t = this.getField("QTY2").value;
var s = this.getField("CS").value;
if (t == 0) event.value = "";
else event.value = s;

not sure if it is correct but the same, it meets my demand.

Please show me the exact javascript codes to me, many thanks!!

[Solved, JR_Boulay 's answer]3. Could anyone tell me, if I finished the form, before I send it to my client, what should I do to prevent others to edit my form while they can sign&stamp on it???

I tried to search the method for the above 3 problems online, but after trying those codes, they didn't work. My bad.

Thank you!