Please help me with 1&2

Amira pulls a 3 pound wagon 4 feet. how much work has she done?

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

When using the scientist method, which step comes last?