Identify the rhombus among the given shapes and calculate the perimeter of a triangle with two sides given.

A rhombus is a quadrilateral (a four-sided polygon) in which all four sides are of equal length. It is also a type of parallelogram, meaning its opposite sides are parallel.

• The first shape is a hexagon (six sides).
• The second shape is a circle.
• The third shape is a triangle (three sides).
• The fourth shape is a quadrilateral with opposite sides marked as parallel (indicated by the arrows). This shape is a parallelogram. Among the given options, this is the only quadrilateral, and a rhombus is a special type of parallelogram. Therefore, this shape represents a rhombus.

The shape that is a rhombus is the parallelogram (the fourth shape).

The perimeter ($P$) of a triangle is the sum of the lengths of its three sides ($s_1, s_2, s_3$).

The perimeter is $25 \text{ cm}$. The lengths of two sides are $7 \text{ cm}$ and $8 \text{ cm}$. The third side is $B$.

First, add the known side lengths:

Now, subtract $15 \text{ cm}$ from both sides to find $B$:

The length of B is $\text{10 cm}$.