kill(all)$

van=facebook.com/LearningCalculus;

f(x):=sqrt(2*a*x^2*(2*a-x)); /* define the function */

fp(x):=''(diff(f(x),x));   /* differentiate function wrt x */

solution:solve(fp(x)=0,x);  /* set derivative to 0 & solve for x */

x1:rhs(first (solution));  /* extract solution 1 */
x2:rhs(second(solution));  /* extract solution 2 */

f(x1); /* build first extremum */
f(x2); /* build second extremem */