Break this up into factors.
9=3x3
12=3x2x2
18=3x3x2
Looking at these factor sets, you need at least 2 3's (for the 9 and 18) and at least 2 2's (for the 12). Since we want the LOWEST multiple, then multiply the factors needed. 2x2x3x3=36.
Hope this helps!
