解:(1)3x^3=51
x^3=17
x=17开3次方
(2)x^2-(169/81)=0
x^2=169/81
x=±13/9
(3)(x-2)^3=8
x-2=2
x=4
(4)3(2-x)^2=27
(2-x)^2=9
2-x=±3
x1=5
x2=-1