The denominators are 16 and 36. Factor them like this
16 = 2 * 2 * 2 * 2
36 = 2 * 2 * 3 * 3
The LCD has to have enough parts to make both 16 and 36, like this
2 * 2 * 2 * 2 * 3 * 3
That contains enough 2's to make 16 and enough 2's and 3's to make 36, so the LCD is
2 * 2 * 2 * 2 * 3 * 3 = 144