Step 1: Find the perimeter of an equilateral triangle. An equilateral triangle has 3 equal sides. If each side is denoted by $l$, the perimeter is the sum of the lengths of its three sides. $$P = l + l + l$$ $$P = 3l$$ The rule for the perimeter of an equilateral triangle is $\boxed{3l}$. Step 2: Find the perimeter of a rhombus. A rhombus has 4 equal sides. If each side is denoted by $l$, the perimeter is the sum of the lengths of its four sides. $$P = l + l + l + l$$ $$P = 4l$$ The perimeter of a rhombus is $\boxed{4l}$. Step 3: Find the perimeter of a regular pentagon. A regular pentagon has 5 equal sides. If each side is denoted by $l$, the perimeter is the sum of the lengths of its five sides. $$P = l + l + l + l + l$$ $$P = 5l$$ The perimeter of a regular pentagon is $\boxed{5l}$. Step 4: Express the associativity of multiplication using variables $a, b,$ and $c$. The associative property of multiplication states that when multiplying three or more numbers, the way the numbers are grouped does not affect the product. Using variables $a, b,$ and $c$: $$(a \times b) \times c = a \times (b \times c)$$ The general way to express the associativity of multiplication is $\boxed{(a \times b) \times c = a \times (b \times c)}$.