It's 1,24 × 10⁸

The original problem was to write $1.24 \times 10^3$ in expanded form.
To expand $1.24 \times 10^3$, you move the decimal point 3 places to the right.

$$1.24 \times 10^3 = 1240$$

If the number were $1.24 \times 10^8$, its expanded form would be 124,000,000.