cap-6.ts

export class AppComponent {
  group = new FormGroup({
    email: new FormControl(),
    password: new FormControl(),
    captcha: new FormControl(),
  })
}

cap-7.html

<form [formGroup]="group" #form="ngForm">
  <input type="email" formControlName="email">
  <input type="password" formControlName="password">

  <app-captcha formControlName="captcha"></app-captcha>
  <div *ngIf="group.get('captcha').hasError('invalid') && form.submitted">
    Wrong answer...
  </div>

  <button type="sumbit">Sumbit</button>
</form>