Given data

• Mass of NaOH = $84\,\text{g}$
• Molar mass of NaOH = $40\,\text{g mol}^{-1}$
• Required molarity = $3\,\text{M}$

Find moles of NaOH $n = \frac{\text{mass}}{\text{molar mass}} = \frac{84}{40} = 2.1\,\text{mol}$

Use molarity relation $M = \frac{n}{V}$ where $V$ is in $\text{dm}^3$.

So, $V = \frac{n}{M} = \frac{2.1}{3} = 0.7\,\text{dm}^3$

Match with the required format We need to write volume as: $\underline{\hspace{2cm}} \times 10^{-1}\,\text{dm}^3$

Since $0.7\,\text{dm}^3 = 7 \times 10^{-1}\,\text{dm}^3$

Final answer The required integer is: $\boxed{7}$

Comparison with stored answer Stored correct answer = $7$

Our derived answer matches the stored answer.