How to hide certain value inside textbox if checked using Javascript?

here I have problem which is I don't know what should I use to solve problem like below:

There have data field inside textbox like

Description of goods : 1 X 40 HQ STC
                       4500 CTNS OF
                       VERMICELLI

Also have checkbox label as "Add Contr STC".

My question is how can be done if, I checked the checkbox, then value of 1 X 40 HQ STCcould be hidden from Description of goodsfield ?

Any ideas for the solution? And I hope you guys will provide some reference if there any because I'm beginner in ASP.net and VB.