Hide left side navigation in SharePoint

you can  hide left side navigation from Site Pages in just few step below

1- Edit Page in SharePoint Designer
2- Go to Insert > Click WebPart > Media and Content > Content Editor Webpart in Page
3- Add below script in your page

<style type="text/css">

body #s4-leftpanel 
{
display:none;
}
.s4-ca 
{
margin-left:0px;
}
</style>

4- Save and see the changes