Answer
$ (3x+4y)(x+1)$
Work Step by Step
$3x^2 + 4xy +3x + 4y = x(3x+4y) + 1 (3x+4y)$
The binomial $(3x+4y)$ is present in both terms and is the greatest common factor. We use the distributive property to factor out $(3x+4y)$
$3x^2 + 4xy +3x + 4y = x(3x+4y) + 1 (3x+4y)= (3x+4y)(x+1)$