Question 316764
Let's plug {{{y=2x-1}}} into {{{x - 3y = 8}}}. 

Then this becomes 
{{{ x - 3(2x-1) = 8 }}} 

Distributing the 3 gives 
{{{ x - 6x + 3 = 8 }}} 

Combining like terms gives 

{{{ -5x + 3 = 8 }}} 

Subtracting 3 from both sides gives 

{{{ -5x = 5 }}} 

Dividing by -5 gives 

{{{ x = -1 }}} 


Now we check: since y = 2x -1, plug in x=-1 to get y = 2(-1) - 1 = -3. Then we check that x - 3y = (-1) - 3(-3) = 8, so this works.