To find how many small cubes can fit inside a larger cube, you need to find the volume of both.
Once you have the volumes, divide the volume of the larger cube, by the volume of the smaller cube.
Volume = L x W x H
Small cube: Volume = 3/4 x 3/4 x 3/4 = 0.421875 cm^3
Large cube: Volume = 3 x 3 x 2 = 18 cm^3
Number of small cubes that can fit: 18 / 0.421875 = 42.67 Cubes.
Since you can't have a partial cube ( 0.67) round down to a whole number:
That means you can fit 42 cubes.