Имеем
x=4-y => (4-y)*y=-5\\y^2-4y-5=0; D=16+20=6*6; y=\frac{4б6}{2} = \left[\begin{array}{ccc}-1\\5\\\end{array}\\;\\\left[\begin{array}{ccc}x+5=4\\x-1=4\end{array}\\;\\\\\left[\begin{array}{ccc}x=-1\\x=5\\\end{array}\\\\" alt="\left \{ {{x+y=4} \atop {x*y=-5}} \right. => x=4-y => (4-y)*y=-5\\y^2-4y-5=0; D=16+20=6*6; y=\frac{4б6}{2} = \left[\begin{array}{ccc}-1\\5\\\end{array}\\;\\\left[\begin{array}{ccc}x+5=4\\x-1=4\end{array}\\;\\\\\left[\begin{array}{ccc}x=-1\\x=5\\\end{array}\\\\" align="absmiddle" class="latex-formula">
Ответ: его корни равны 5 и -1