Convert 7.5 meters to millimeters.

To solve this problem, we need to convert the length from metres (m) to millimetres (mm).

Step 1: Understand the conversion rule.
There are 1,000 millimetres in 1 metre.
So, to change metres into millimetres, you multiply by 1,000.

Step 2: Set up the calculation.
The length of the classroom is $7.5$ m.
$$7.5 \times 1,000 = ?$$

Step 3: Do the math.
When you multiply a decimal number by 1,000, you move the decimal point three places to the right.
* Start with: $7.5$
* Move once: $75.$
* Move twice: $750.$
* Move three times: $7500.$

So, $7.5 \text{ m} = 7,500 \text{ mm}$.

Final Answer:
7500 mm