I need to erase to iphone using my computer. My phone is broken.

Unless you can access your iPhone, you will not be able to erase it ... whether or not, you use a computer to do so. You do have the option to contact Apple Support directly to see if there is anything they can do to assist you with this matter. To do so, simply select the "Get Support" link in the upper right-hand side of this forum's web page.