>>350
x^6とx^3が出てるのだから
わからなければ
t = x^3
とでもおいて

x^6 + 7x^3 -8 = t^2 +7t -8 = (t+8)(t-1)

t+8 = x^3 +8 = x^3 +2^3 = (x+2)(x^2 -2x+4)
t-1 = x^3 -1 = (x-1)(x^2 +x+1)