Geometry

Question Analysis:
This question is about using the diagonal properties of a kite to solve for an unknown variable.
The main focus is understanding that the longer diagonal bisects the shorter diagonal and applying basic algebra.

Key Concept Explanation:
In any kite:
The longer diagonal bisects the shorter diagonal at a right angle.
This means the shorter diagonal is cut into two equal parts.
If one half is given, multiply by 2 to find the full length, then solve for the variable.

Step-by-Step Solution:
We are given that half of BD is 9 cm.
Therefore, the full length of BD is: 9 × 2 = 18 cm.
Set up the equation using the given expression: 3x + 3 = 18
Solve for x:
Subtract 3 from both sides: 3x = 15
Divide by 3: x = 5