It appears you have a couple of emails from your Yahoo email account that won't complete downloading in the Mail application on your iPad. You'd like to get rid of them, but you're unable to. I'm happy to help.
How to force an app to close on your iPhone, iPad, or iPod touch -- First, force close Mail application if it's open, then -- Restart your iPhone, iPad, or iPod touch -- Restart your iPad. Once it's returned to the Home screen, test the issue.
Set up an email account on your iPhone, iPad, or iPod touch -- If the issue persists, tap Settings > Accounts & Passwords and select the Yahoo account and delete it. (If this is a POP3 email account, then you can only check email on one device at a time. Close other email programs and sign out of webmail sites that you have open. To check if your email is a POP3 account, go to Settings > Accounts & Passwords, tap your account, tap Account, then look for the label POP Account Information.) Once the account has been deleted, add it back again and see if those emails have completed downloading or have disappeared.
Let me know how that goes.