Question 27654
3x-3y=18  
x+y=3

Multiplying the 2nd equation with 3
3x+3y = 9
Now, adding both
6x = 27
{{{X = 27/6}}}
X = 4.5
Putting this  value back into any equation we get Y = -1.5

*[invoke linear "x", "y", 3, -3, 18, 1, 1, 3]

*[invoke linear_substitution "x", "y", 3, -3, 18, 1, 1, 3]