How To use Extensions In Visualforce page in Salesforce

Visualforce page

<apex:page standardController="Account" extensions="myextension">
    <p>{!title}</p>   
    <apex:form >
        <apex:inputField value="{!account.name}"/>

        <apex:commandButton value="Save" action="{!save}"/>
    </apex:form>
</apex:page>

extensions

public class myextension {
    private final account ac;
   
    public myextension(Apexpages.standardcontroller controller){
        this.ac =(Account)controller.getRecord();
    }
    public String getTitle(){
        return 'Account:' + ac.name + '(' + ac.id + ')';

    }
}