Find the total water volume from both cubes, then divide by the tank base area.<br><br>
&bull; Cube 1 (side 12 cm): volume = 12&sup3; = 1728 cm&sup3;<br>
&bull; Cube 2 (side 6 cm): volume = 6&sup3; = 216 cm&sup3;<br>
&bull; Total water = 1728 + 216 = 1944 cm&sup3;<br>
&bull; Tank base area = 12 × 9 = 108 cm&sup2;<br>
&bull; Water height = 1944 ÷ 108 = 18 cm<br><br>
Final Answer: 18 cm