20 cm the answer is 20 cm
Th area of a right triangle is A = 1/2 (ab), where a and b are the legs of the right angle. So area of the base is, A = 1/2 (9 x 12) = 108 cm2. With 2 bases, total area of base is 108 times 2, which is equal to 216 cm2. The other side of the triangular base nypotenuse) can be computed by Phythagorean theorem. Hypotenuse = square root of sum of the suare of 9 and 12. This is equal to 15 cm. There are other 3 rectangular faces with the following dimensions: 9 by 10 cm, 12 by 10 cm and 15 by 10 cm. Area of the other faces are 90, 120 and 150 cm2, by multiplying the length and width, only. Total area is 576 cm2.
The volume of the new rectangular prism would be 270 cubic centimeters. The reason for this is that when only one dimension is changed, the overall volume is only changed by that same relationship. Because only the height was cut in half, this would be only one of the dimensions.