[lubuntu] Formating a encrypted hard drive on mac

A few weeks ago, I encrypted my macbook air with filevault. Since then, I have forgotten the password I used for filevault.
So, I burned ubuntu onto a usb and installed it on my macbook. I have been trying to format my internal hardrive however, everytime it is telling me to insert the password and my idiot self has forgotten it. Any help I can use to format my encrypted internal hard drive?

p.s. I'm a beginner when it comes to ubuntu.

Thanks,

Rizman