please explain steps on how to solve

Chloe$$s Cafe bakes croissants that it sells to local restaurants and grocery stores. The average costs to bake the croissants are $$0\mathrm{.}70$ for $3,900$ and $$0\mathrm{.}65$ for $7,800.$

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Required:

If the total cost function for croissants is linear, what will be the average cost to bake $7,000?($Do not round intermediate calculations. Round your final answer to $4$ decimal places.$)$