Solution:
              f(-2): evaluate f(x)  for  x = -2
              Insert the value of x and do the math operations.

              f(x) = 2x³ - 8x   if  x = -2  then:       
              f(-2) = 2(-2)³ - 8(-2)  
              f(-2) = 2(-2)³ + 16  
              f(-2) = 2(-8) + 16
              f(-2) = -16 + 16
              f(-2) =  0