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 $\boxed{3l}$.

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 $\boxed{4l}$.

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 $\boxed{5l}$.

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 $\boxed{(a \times b) \times c = a \times (b \times c)}$.