Method 1:
\Factor the each monomial.
\18k = 2 · 3 · 3 · k
\15k3 = 3 · 5 · k · k · k \ \
\The GCF is the product of the common prime factors in each monomial.
\GCF = 3 · k
\The greatest common factor of 18k and 15k3 is 3k.
The greatest common factor of 18k and 15k3 is 3k.