The first expression is
8y^4 + 4y^3 + 16y - 8y
The greatest common factor is the highest factor common to each all the terms. For this expression, the GCF is
4 and y because 4 is the highest number that all the terms can divide without a fraction and a single y is common to all that terms. Thus,
GCF = 4y
The second expression is
3x^4 + x^3 - 3 + 3x
There is no number or term that is common to all the terms in the expression. Thus
No GCF