How can I multiply 'checkbox1' value with 'textbox1' value and get the result of it in 'textbox2'

Although script is not correct (it will give you NaN) but if text field has value and checkbox is checked it should still calculate.

Are you sure your field names are correct? Because in script you use "Text1" and "Checkbox1" but in your second post you refer to "textbox1" and "checkbox1", check console for error.

To get rid of NaN error check that checkbox is checked before you use calculation script.

EDIT:

Try this script as custom calculation script of 'textbox2' field and make sure field names are correct:

var a = Number(this.getField("Text1").valueAsString);
var b = this.getField("Checkbox1");
if(b.valueAsString != "Off")
event.value = Math.abs(a)*Number(b.valueAsString);
else
event.value = "";