Алгебра: Вопрос в картинках...

Решите задачу:

$(x+1)(x+2)(x+3)(x+4)=15\\ zamena\\ x+1=t\\ t(t+1)(t+2)(t+3)=15\\ t^4+6t^3+11t^2+6t-15=0\\ t^4+3t^3+5t^2+3t^3+6t^2+6t-15=0\\ t^2(t^2+3t+5)+3t^3+6t^2+6t-15=0\\ t^2(t^2+3t+5)+3(t^3+6t^2+6t-15)=0\\ o4evidno\ t=1\togda\\ t^2(t^2+3t+5)+3(t-1)(t^2+3t+5)=0\\ (t^2+3t-3)(t^2+3t+5)=0\\ t=\frac{-\sqrt{21}-3}{2};\\ t_{2}=\frac{\sqrt{21}-3}{2};\\ x+1=t\\ x= \frac{-\sqrt{21}-3}{2};\\ x=\frac{\sqrt{21}-3}{2};\\$