Least common multiple

Product of two monomials $A$ and $B$ is

$AB=72x^5$

Their greatest common divisor is $6x^2$ , so they can be written as

$A=a\times 6x^2$

$B=b\times 6x^2$

And the product then is $AB=ab(6x^2)^2=36abx^4$

which has to be equal to $72x^5$ , giving us an equation for $ab$

$36x^4ab=72x^5$

with the solution

$ab=2x$

There is not enough information to obtain $a$ and $b$ , and therefore $A$ and $B$ , individually, but their least common multiple will be

$2x\times6x^2=\boxed{12x^3}$