Question: 13. Find the missing side length of the rectangle $d$.
$P = 18$ cm
$3$ cm
$d$
rectangle at center: width labeled $d$ on bottom, height labeled $3$ cm on right, perimeter $P = 18$ cm inside; position_hint=center
1. **State the problem:** We need to find the missing side length $d$ of a rectangle given its perimeter $P = 18$ cm and one side length $3$ cm.
2. **Formula for perimeter of a rectangle:**
$$P = 2 \times (\text{length} + \text{width})$$
3. **Assign known values:**
Let the length be $d$ and the width be $3$ cm.
4. **Write the equation:**
$$18 = 2 \times (d + 3)$$
5. **Divide both sides by 2:**
$$\frac{18}{2} = \cancel{2} \times (d + 3) / \cancel{2}$$
$$9 = d + 3$$
6. **Solve for $d$:**
$$d = 9 - 3$$
$$d = 6$$
7. **Answer:** The missing side length $d$ is $6$ cm.