kill(all)$

van=facebook.com/LearningCalculus;

f(a):= a-a*sqrt(a)-12 = 0;

f(x);

x1:(-3/2)*%i*(sqrt(15)-%i);

x2:(3/2)*%i*(sqrt(15)+%i);

radcan(realpart(f(x1))); radcan(imagpart(f(x1)));
radcan(realpart(f(x2))); radcan(imagpart(f(x2)));