<image> A store restocked a batch of tea at a unit price of $$40$$ yuan$$/$$kilogram. According to a survey, during a certain period, the sales volume $$y($$kilograms$$)$$ and the sales unit price $$x($$yuan$$/$$kilogram$$)$$ have the functional relationship as shown in the figure.
$$(1)$$ Derive the functional relationship between $$y$$ and $$x$$ from the graph; 
$$(2)$$ The store wants to achieve a sales profit of $$2400$$ yuan while ensuring that the sales cost does not exceed $$3000$$ yuan. What should be the sales unit price?