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7. Add or subtract the following polynomials:
a. 5x2 + 3y3 - 4x2 =
b. y3 + 2y3 - 8 + 2 =
c. 9-3x - 4x + 2x - 12 =
d. -6t + 4+2 -16 + 7+ - 4+2 =