I assume that each cell divides into 2 when it divides. However many cells we have at any time, six hours later we have twice that number. So

initially: 1 cell

after six hours: 2 cells

after 12 hours: 2x2 = 4 cells

after 18 hours: 2x4 = 8 cells

after 24 hours: 2x8 = 16 cells

...

after 6n hours: 2x2x...x2 (n times) = 2^n cells