Question 173015
Hope these additional  comments help.
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y=5-4x 
Now replace y with(5-4x) in the other equation below & solve for x.
3x-2y=12
3x-2(5-4x)=12 multiply 2(5-4x)
3x-10+8x=12 add (3x8x, add 10 to both sides of the equation)
11x=12+10 add the (12+10)
11x=22 (now divide both sides by 11)
x=22/11 (divide 22 by 11)
x=2 ans. (now substitute 2 for x in the first equation & solve for y)
y=5-4*2 (multiply -4*2)
y=5-8 (subtract 8 from 5)
y=-3 ans.
Proof: (replace x with 2 & y with -3 & solve the equation)
3*2-2*-3=12
6+6=12