Найти точку минимума функции у=х³-48х+17

Y(x)=x³-48x+17
y`(x)=3x²-48=3(x²-16)=3(x-4)(x+4)
y`(x)=0 при 3(x-4)(x+4)=0
                            +                                      -                               +
                 -44
                                         max                                      min
x(min)=4