Question 874383
X^2+y^2=10
X+3y=10

x=10-3y

substitute

{{{(10-3y)^2+y^2 =10}}}

{{{100-60y+9y^2+y^2=10}}}


{{{10y^2-60y+90=0}}}


/10

y^2-6y+9=0

y^2-3y-3y+9=0

y(y-3)-3(y-3)=0

(y-3)^2=0
y-3=0
y=3