first, the numbers
lcm of 15, 5, and 21
15 = 3 * 5
5 = 5
21 = 3 * 7
lcm is 3 * 5 * 7 because 3 * 5 * 7 has 3 * 5 (to make 15), a 5 (to make 5), and 3 * 7 (to make 21)
3 * 5 * 7 = 105
lcm of 15, 5, and 21 is 105
now the b's
the lcm of b^2, b^5, and b^5 is b^5
note b^5 = b^2 * b^3
now the a's
the lcm of a^5, a^3, and a^6 is a^6
the lcm of the whole thing is 105(b^5)(a^6)