Question

left { {{x^2+y^2=28} atop {x+y=4}} right.

Answer 1

$left { {{x^2+y^2=28} atop {y=4-x}} right. \ left { {{y=4-x} atop {x^2-4x-6=0}} right. \ left { {{x= 2+ sqrt{10} } } atop {y=2-sqrt{10}}} right. ---or--- left { {{x=2-sqrt{10}} atop {y=2+sqrt{10}}} right.$