Question 1171574
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The 1st equation is x = 3y
It means x is the same as 3y. 
Whenever we see "x", we can replace it with "3y".


We'll do so in the 2nd equation.
x-3y = 0
<font color=red>x</font>-3y = 0
<font color=red>3y</font>-3y = 0
0 = 0
We get the same thing on both sides. 
Therefore, we have <font color= blue>infinitely many solutions</font>


This is the case because x = 3y can be rearranged to x-3y = 0, and vice versa.
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