"To get that $1.4 billion in revenue, they will need to sell 40k Tesla 3s at $35k each, or 3,000 a week. To get it in margin, even assuming the average model 3 will sell for $50k, they will have to sell 12,000 a week or so (at 20% margin)"
It seems like you're assuming that the cash burn will stay at 1.4billion. Expenditure should go down once the production line tooling is complete.