Final answer:
The expression representing the number of sides the second polygon has, when given a polygon with n sides, is 2n. This is found by multiplying the number of sides of the first polygon by two.
Step-by-step explanation:
If a polygon has n sides, then a second polygon with twice as many sides would have 2n sides. This expression 2n represents the number of sides of the second polygon. To understand this better, let's consider an example: if the first polygon is a triangle (n = 3), then the second polygon would be a hexagon with 6 sides (2 x 3 = 6). Therefore, the expression that represents the number of sides the second polygon has, when the first polygon has n sides, is simply 2n.