用韦达定理算方程x^2+4x-a=0的一根是另一根的2倍,求a的值,并求出两根.

assume x1=2x2 sincex1+x2=-4 we get3x2=-4 that isx2=-4/3 x1=-8/3 and sincex1*x2=a we obtain the result a=32/9