Answer:
7x (x + 1 ) ( x + 3 )
Step-by-step explanation:
7x³ +28x² +21x
7x ( x² + 4x + 3 )
7x [ x² + 1x + 3x + 3 ]
7x [ x ( x + 1 ) + 3 ( x + 1 ) ]
The answer is C; 6