First factor the each term in the expression.

6 =  2 · 3

3y =  3 · y

The GCF is the product of the common prime factors in each term.

GCF = 3

Write each term as a product of the GCF and its remaining factors.

6 + 3y = 3(2) + 3(y)

Combine like terms uisng distribuitve property: ab + ac = a(b + c).

6 + 3y = 3(2 + y)

Simplified expression is 3(2 + y).