18. b) The density of platinum at \( 400^{\circ} \mathrm{C} \) is \( 2120 \mathrm{~kg} \mathrm{~m}^{-3} \). Find its density at \( 30^{\circ} \mathrm{C} \). Given, coefficient of linear expansion of platinum is \( 9.0 \times 10^{-6} \mathbf{K}^{-4} \).
[WBSCTE 2018]