Question 252192
In an (x, y) coordinate system, write the equation of the vertical line passing
through the point of intersection of 3x + 4y = 1 and x + 3y = 7 .
.
First, find out where the two lines intersect:
3x + 4y = 1 
 x + 3y = 7
.
Multiply second equation by -3:
 3x + 4y = 1 
-3x - 9y = -21
.
Add the two equations together:
 3x + 4y = 1 
-3x - 9y = -21
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    -5y  = -20
      y  = 4
.
Use the second equation to find x:
 x + 3y = 7
 x + 3(4) = 7
 x + 12 = 7
 x = -5  (which is also the vertical line they are looking for)