Answer
4
Work Step by Step
Find the greatest common factor. Find the prime factors of each term and multiply the factors that are common to all the terms.
$4x^3=\underline2\times\underline2\times x\times x\times x$
$12x=3\times\underline2\times\underline2\times x$
$28=7\times\underline2\times\underline2$
GCF $=2\times2=4$