Question 508704
{{{x*y=21}}}
{{{x+y=10}}}
1) -y on both sides
{{{x= 10-y}}}
2) plug x in
{{{x*y=21}}}
{{{(10-y)*y=21}}}
3) Distribute
{{{-y^2+10y=21}}}
4) Move 21 over
{{{-y^2+10y-21=0}}}
5) Factor
{{{(y-3)(-y+7)}}}
6) Y = 3 or 7
7) Thus X = 3 or 7 depending on Y 
8) Check
{{{(3)*(7)=21}}}
{{{(3)+(7)=10}}}