x6-26x3-27=(x3-27)(x3+1), so x3=27, x=3 (plus two complex roots), or x3=1, x=1 (plus two complex roots)
COMPLEX ROOTS
x3-27=(x-3)(x2+3x+9)⇒x=(-3±√(9-36))/2=(-3±3i√3)/2;
x3-1=(x-1)(x2+x+1)⇒x=(-1±√(1-4))/2=(-1±i√3)/2.
FULL SOLUTION (6 roots)
x=1, 3, (-1+i√3)/2, (-1-i√3)/2, (-3+3i√3)/2, (-3-3i√3)/2.