The straight rods AB and CD cross at the point O, which is the middle point of both…

Step 1. The triangles. Take $\triangle AOC$ and $\triangle BOD$.

Step 2. The equal parts.

1. $OA = OB$, because $O$ is the middle point of $AB$.
2. $\angle AOC = \angle BOD$, because they are vertically opposite angles.
3. $OC = OD$, because $O$ is the middle point of $CD$.

Step 3. The rule. $\triangle AOC \cong \triangle BOD$ by SAS, with $A \to B$, $O \to O$ and $C \to D$. So $AC = BD$ by CPCT.

Step 4. The number. $BD = AC = 7\ \text{cm}$.