Java Script - check box in adobe to (When checked) to subtract value

Im trying to get a check box in adobe to (When checked) to subtract "TTLCalculated" And use that value in the next part of my code, wanted to know if what im doing wrong, im a newbie to javascript, would really appreciate the help!

var DA = Number(this.getField("Downpayment*").valueAsString);
var DA2 = Number(this.getField("TTLCalculated").valueAsString);
var DA3 = Number(this.getField("TaxTagsIncluded").valueAsString);
If “TaxTagsIncluded” = True
event.value = (DA-DA2);
else;
Return;