import { NgModule }       from '@angular/core';
import { BrowserModule }  from '@angular/platform-browser';
import { FormsModule }    from '@angular/forms'
import { AppComponent }   from './app.component';
import { HeroDetailComponent } from './hero-detail.component';

@NgModule({
  imports:      [ BrowserModule, FormsModule],
  declarations: [ AppComponent, HeroDetailComponent ],
  bootstrap:    [ AppComponent ]
})
export class AppModule { }

import { NgModule }       from '@angular/core';
import { BrowserModule }  from '@angular/platform-browser';
import { FormsModule }    from '@angular/forms'
import { AppComponent }   from './app.component';
@NgModule({
  imports:      [ BrowserModule, FormsModule],
  declarations: [ AppComponent ],
  bootstrap:    [ AppComponent ]
})
export class AppModule { }

--- 
+++ 
@@ -2,9 +2,11 @@
 import { BrowserModule }  from '@angular/platform-browser';
 import { FormsModule }    from '@angular/forms'
 import { AppComponent }   from './app.component';
+import { HeroDetailComponent } from './hero-detail.component';
+
 @NgModule({
   imports:      [ BrowserModule, FormsModule],
-  declarations: [ AppComponent ],
+  declarations: [ AppComponent, HeroDetailComponent ],
   bootstrap:    [ AppComponent ]
 })
 export class AppModule { }
Add HeroDetailComponent to the NgModule
