4a^2b and 6ab^3
first, find the greatest common factor of ur coefficients 4 and 6.....and that is gonna be 2
as for ur variables and exponents....take the lowest ones
so both have an " a " and a " b " variable....the lowest a term is just " a "...and the lowest " b " term is b
so the GCF of these 2 terms are : 2ab