帮忙解个数学题：已知X的平方+2X—1=0,求X的三次方—2X+4的值

x^2 + 2x -1 = 0 (x+1)^2 =2
x^3 - 2x + 4
=(x+1)^3 -3(x+1)^2 + (x+1) + 5
= 正负3倍根号2 -1