Question 188345
I do not know where to start. When do you multiply numbers to the x and y.
my problem is this 
2x+y = 5
-6x - 3y = -15 My teacher said that you multiply the 2x and the y by 3 why? and how do you finish after that?
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In order to eliminate one of the variables, you multiply one of the equations by something to get the same coefficients.  In this case, multiplying the 1st eqn by 3 gives 6x + 3y = 15.  The coeff's for x and y are the same as in the other equation.  That's the reason to multiply by 3.
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In this example, tho, the 2 equations are actually the same.  If you add them (after multiplying by 3) you get 0x + 0y = 0.
There is no unique solution for this pair of eqns.
If you graph them, you get the same line 2 times.