Typehint argument with CalculatorService if autowiring is enabled or pass it as argument of your service in configuration file.
use Alapin\CalculatorBundle\Service\CalculatorService;
public function showResult(CalculatorService $calculatorService)
{
    $expression = '(2.16 - 48.34)*2.7';
    $result = $calculatorService->calculate($expression, 'default')
    
    return $result;
}