A stationary speed gun emits a microwave beam at 2.10x10^10 Hz. It reflects off a car and returns 638 Hz lower. What is the speed of the car? (Unit=m/s)

Assume you measured the mass of the cart to be (500 ± 1) g and the mass of the additional mass you put on the cart to be (500 ± 1) g as well. since the scale you are using in the lab cannot measure objects heavier than 600g you will have to sum up individual pieces and propagate the error. so what would be the mass and the standard error of the cart and the mass