x^4-4x^3-4x^2+15x+18=0x^4-3x^3-x^3+3x^2-7x^2+21x-6x+18=0(x-3)(x^3-x^2-7x-6)=0x1=3x2=3.49449283705542x3=-1.24724641852771-0.40170103640146ix4=-1.24724641852771+0.40170103640146i