do those 2 equaiton you did
minus x on top
y=10-x
sub for y
x(10-x)=12
distribute
10x-x^2=12
minus (10x-x^2) both sides
0=x^2-10x+12
use quadratic formula
in form
ax^2+bx+c=0
x=

a=1
b=-10
c=12
x=

x=

x=

x=

x=

x=

or

aprox
x=1.39445 or 8.60555
those are the 2 numbers since x and y can switch