x+y=21 subtract x form both sides y=21-x subsitute x(21-x)=110 21x-x^2=110 add x^2-21x to both sides 0=x^2-21x+110 use quaratic formula which is (-b+/-√(b^2-4ac)/(2a)=x when you have it in the format of ax^2+bx+c=0 so
a=1 b=-21 c=110
input two equations (-(-21)+√(-21^2-4 times 1 times 110))/(2) or (-(-21)+√(-21^2-4 times 1 times 110))/2