Question

How to solve #4 using substitution/ elimination method

Answer 1

3x+y = 4
  x+y=  2 multiply all the elements of (x+y) by -1 & then add it up to the 1st equation
3x+y=4
-x-y =-2
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2x+0y =2 ==> x=2/2 =1 & replace x=1 by its value in any of the equation & you will get the value of y=1

2) 3x+y = 7
x+2y = 4
Follow the same methodology as before, but to eliminate y, multiply the 1st equation by (-2) and you will get x=2 & y=1