Answer
See below.
Work Step by Step
We will need to prove two statements:
Proof1: "for all positive integers a and b, if a | b then lcm(a, b) = b."
As a|b, let b=ka, we have lcm(a,b)=lcm(a, ka)=ka=b.
Proof2: "for all positive integers a and b, if lcm(a, b) = b, then a | b."
As lcm(a, b) = b, we have $b$ as multiplies of each component, in other words $b=ka$, thus $a|b$.
