$$z^3 = -18 * j - 26$$$$|z^3|= \sqrt{18^2+26^2} ≈31,6228$$$$|z|=(18^2+26^2)^{1/6}≈3,1623$$$$φ=arcsin( -18/ \sqrt{18^2+26^2})≈214,695°$$$$φ_1=φ/3≈71,565°$$$$φ_2=φ_1+120≈191,565°$$$$φ_3=φ_1+240≈311,565°$$$$z_i=|z| *(cos ( φ_i)+j sin (φ_i))$$$$z_1≈3,1623*(cos ( 71,565°)+j sin (71,565°))$$$$z_2≈3,1623*(cos ( 191,565°)+j sin (191,565°))$$$$z_2≈3,1623*(cos ( 311,565°)+j sin (311,565°))$$