How do I create the calculation to subtract one text box from another?

Thank you for the information.  I tried both the Simplified field notation (text12-text17) and (text12-text17=text18) and the custom calculation script coding and both ways give me the options to "cancel" or to "go to...." which then wants a "Line Number:"  I don't have a line number.  I tried 18, text18, & 7. (the number in the document that has the place for the answer)  Nothing seems to work.  When I click "ok", nothing happens.

Lee

You may use built-in features, such as simplified notation in the field where you desired the event or action to take place; "Text18".

See the brief discussion in the link posted below: 

• https://answers.acrobatusers.com/How-perform-calculations-requiring-subtraction-q22253.aspx

Or you may also employ a custom calculation script in the field "Text18".

var a = this.getField("Text12").value;

var b = this.getField("Text17").value;

if (a !=="" || b !=="") {

event.value = a-b;

} else {

event.value ="";

}

The custom script above is just one out of many examples that you can use to obtain the same result.

It may have errors and I haven't tested it on my end (typing from a mobile device).