39gii bizarre graph extremum

[lrdheat]

When plotting x^(2/3) * (6-x)^(1/3), the function plots fine (I chose to plot from -.5 to 6.5, and then used auto scale). When I asked for extremum, it comes up with a point well to the right of the extremum! The actual extremum is (4,~3.17+). The 39gii plot places it at (~4.98,~2.94)!

When I go to solve and take d/dx of the function to be zero, it correctly comes up with 3.9999999999 (4).

Area under curve between roots (which it finds correctly) impossible.