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18x - 12y = 24 (1)
-6x + 4y = 12 (2)
To reduce coefficients (and make your solution easier), divide eq(1) by 6 (both sides); divide eq(2) by -2 (both sides).
You will get
3x - 2y = 4 (3)
3x - 2y = -12 (4)
From this point, you can just see that the system is inconsistent: left sides of equations (3) and (4) are identical,
while their right sides are different. So, the system has no solution/solutions.
But I will follow instructions and will apply the substitution method.
Let's see what will happen.
From equation (3), I express 3x = 4 + 2y and substitute it into equation (4). I get
(4 + 2y) - 2y = -12
2y - 2y = -12 - 4
0 = -16. (*)
Thus we got self-contradictory equation (*), which means that the original system has no solution/solutions.
It confirms our conclusion, which was made earlier.
Solved.