Question 327031
{{{x + y = 3}}}
{{{y = x + 3}}} = {{{x -y = -3}}}
You have two lines. Not the same and not parallel. So one solution exsits.

Using elminatiion
{{{x + y = 3}}}
{{{x - y = -3}}}  add the two
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{{{2x = 0}}}
{{{x = 0}}}
If x - 0, then use that to find y
{{{x + y = 3}}}
{{{0 + y = 3}}}
y = 3

Plot and verify graphically that the point of intersection is (0,3)
{{{graph(400,400,-10,10,-10,10, 3-x, x + 3)}}}