GCF of 42 and 90
a simple way to find the GCF is to decompose 42 & 90 into appropriate factors:
42 = 2 x 3 x 7 & 90 = 2 x 3² x 5
What is the GCF of the above: It's the product of COMMON FACTORS raised at the smallest power:
COMMON FACTOR are 2 x 3 (these are the COMMON raised to the smallest power. Notice I disregarded 3²) so the GCF = 2 x 3 = 6