a) x^3 * x^2 - 2x * x * x^3 + x^7:x^2 = x^5 - 2x^5 + x^5 = 0
b) y^5 * y^4 - 2y * y^13:y^5 + 3y^12:y^3 = y^9 - 2y * y^8 + 3y^11 = y^9 - 2y^9 + 3y^11 = -y^9 + 3y^11 = y^9(-1+3y^2) = y^9(3y^2-1)
c) a^20:(-a)^16 + 5a*(-a)^3 + (-2a^2)^2 = a^20:a^16 - 5a^4 + 4a^4 = a^4 - 5a^4 + 4a^4 = 0