Here's the entire class (I preserved the other methods from a previous Trailhead module, the method in question here is the first one - sendMail):

public with sharing class EmailManager 
{
    public static void sendMail(String address, String subject, String body)
    {
        Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
        String[] toAddresses = new String[] { address };
        mail.setToAddresses(toAddresses);
        mail.setSubject(subject);
        mail.setPlainTextBody(body);
        Messaging.sendEmail(new Messaging.SingleEmailMessage[] { mail });
    }
    
    // Public method.
    public static void sendEmail(String address, String subject, String body) {
        
        // Create an email message object.
        Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
        String[] toAddresses = new String[] {address};
        mail.setToAddresses(toAddresses);
        mail.setSubject(subject);
        mail.setPlainTextBody(body);
        
        // Pass this email message to the built-in sendEmail method
        // of the Messaging class.
        Messaging.SendEmailResult[] results = Messaging.sendEmail(new Messaging.SingleEmailMessage[] {mail});
        
        // Call a helper method to inspect the returned results.
        inspectResults(results);
    }
    
    private static Boolean inspectResults(Messaging.SendEmailResult[] results)
    {
        Boolean sendResult = true;
        
        // sendEmail returns an array of result objects.
        // Iterate through the list to inspect results.
        // In this case, the methods send only one email,
        // so we should only have one result.
        
        for(Messaging.SendEmailResult res : results) {
            if(res.isSuccess()) {
                System.debug('Email sent successfully');
            }
            else {
                sendResult = false;
                System.debug('The following errors occurred: ' + res.getErrors());
            }
        }
            
        return sendResult;
    }
}