First create a box, add plenty of segments and collaps it to an editable mesh, select the back and side faces and delete these to give you your first poly‘s.



Then move the vertexes in place, giving a ruff outline of the chassi.



Repeat in the top view.



Select the top row of edges, hit shift and drag them up a little bit, switch to top view and drag them inwards a bit, this will give the chassi a rounded look.



Now shift and move twice along the x axis to the middle of the car, you can align vertexes by selecting them and use view align .