Answer:
See below.
Step-by-step explanation:
i) A + B + C
= 4x² + y² - 6xy + 3y² + 12x² + 8xy + 6x² + 8y² + 6xy
= 24x² + 12y² + 8xy
ii) 2A + B - C
= 2(4x² + y² - 6xy) + 3y² + 12x² + 8xy - 6x² - 8y² - 6xy
= 8x² + 2y² - 12xy + 3y² + 12x² + 8xy - 6x² - 8y² - 6xy
= 14x² - 3y² - 10xy
