Find the total water volume from both tanks, then divide by the new tank's base area.<br><br>
&bull; Tank 1: 24 × 10 × 9 = 2160 cm&sup3;<br>
&bull; Tank 2: 18 × 12 × 5 = 1080 cm&sup3;<br>
&bull; Total water = 2160 + 1080 = 3240 cm&sup3;<br>
&bull; New tank base area = 12 × 9 = 108 cm&sup2;<br>
&bull; Height = 3240 ÷ 108 = 30 cm<br><br>
Final Answer: 30 cm