a: \(=\dfrac{12x^7+48x^5-24x^4+63x^3+252x-126}{4x^4+21}\)
\(=\dfrac{12x^7+63x^3+48x^5+252x-24x^4-126}{4x^4+21}\)
=3x^3+12x-6
b: \(=\dfrac{20x^6+16x^5+32x^4+24x^3-47x^2-40x-5}{2x^4+3x^2-5}\)
\(=\dfrac{20x^6+30x^4-50x^2+16x^5+24x^3-40x+2x^4+3x^2-5}{2x^4+3x^2-5}\)
=10x^2+8x+1