cap-3.ts

@Component({...})
export class CaptchaComponent implements OnInit, ControlValueAccessor {
  @ViewChild('canvas', { static: true }) canvas: ElementRef<HTMLCanvasElement>;
  answer: number;

  ngOnInit() { this.createCaptcha(); }

  ...
    
  createCaptcha() {
    const ctx = this.canvas.nativeElement.getContext("2d");
    const [numOne, numTwo] = [random(), random()];
    this.answer = numOne + numTwo;

    ctx.font = "30px Arial";
    ctx.fillText(`${numOne} + ${numTwo} = `, 10, 35);
  }

  change(value: string) {
    this.onChange(value);
    this.onTouched();
  }

}

function random() {
  return Math.floor(Math.random() * 10) + 1;
}