Answer:
C
Step-by-step explanation:
line up 'like terms' below each other:
5[tex]x^{2}[/tex] + 9x + 3
- 6[tex]x^{2}[/tex] + 3x + 0
remember, when you subtract then you 'add the opposite'
5[tex]x^{2}[/tex] + 9x + 3
+ -6[tex]x^{2}[/tex] + 3x - 0
-[tex]x^{2}[/tex] + 12x + 3
