Let there be x*y cubes on the bottom. Perimeter = 2(x + y)
2(x + y) = 18; x + y = 9
Possibilities
x = 8, y = 1. 8 cubes on the bottom. But 42/8 is not a whole number, so not this.
x = 7, y = 2. 14 cubes on the bottom. 42/14 = 3, so height = 3 cubes = 3 cm.
x = 6, y = 3. 18 cubes on the bottom. 42/18 not a whole number.
x = 5, y = 4. 20 cubes on the bottom. 42/20 ...etc.