what is the simplified form of each expression? (-2x^8) - 3y^9 - 2x^4 A 3x^12 y^9 b -12x^72 y^9 c-12xy^21 d -12x^12 y^9 5 find the simplified form of the expression. give your answer in scientific notation. (7x10^-4) (9x10^-10) A6.3x10^-13 b 63x10^-13 c 6.3x10^-15 d6.3x10^-16