Trying To Make A Check Box "Force" 2 text fields to be overwritten

Trying To Make A Check Box "Force" 2 text fields to be overwritten in adobe using (java script) when check box is checked,

Im attempting to use this horribly written Java Script... I altered a number normula for string format (please dont judge..), I have limited knowledge of javascript and would appreciate a re-write of the code below:

var A12 = this.getField("Co Buyer Street Address").valueAsString); 
var A13 = this.getField("Co Buyer City, State, Zip").valueAsString);
event.value = A12 == Street Address;
event.value = A13 == Buyer City, State, Zip;