The least common multiple can be found a couple of ways. For this, it is probably easiest to identify the unique factors of each expression.
(6x -18) = 6(x -3) = 2*3*(x -3)
(2x² -6x) = 2x(x -3)
Then the unique factors are 2, 3, x, (x -3). Their product is the LCM:
LCM = 6x(x -3) = 6x² -18x
_____
If one or more of the factors is raised to a power other than 1, use the highest power of that factor in the final product.