Question 143504
plot y=6-x and y=x-3  and see the place where they intersect.

{{{graph( 300, 200, -10, 10, -10, 10, 6-x, x-3 )}}}

Looks like y=3/2 and x=9/2.

Check via substitution:
*[invoke linear_substitution "x", "y", 1, 1, 6, 1, -1, 3 ]