Let the pen cost 1 unit. Book costs 6 units. Total = 7 units = \(x\).<br><br>&bull; Pen = 1 unit, Book = 6 units<br>&bull; Total = 7 units = \(\$x\)<br>&bull; 1 unit = \(\frac{x}{7}\)<br>&bull; Pen = \(\frac{x}{7}\)<br><br>Final Answer: D: \(\frac{x}{7}\)