Question 1127777
<pre>
(1)   3x + 6y = 21  
(2)   3x + 3y = 15
</pre>

This system can be quickly solved for y by, say,  subtracting eq(2) from eq(1):
 <pre>
                3y = 6
                 y = 2  —>  3x = 21-12 = 9 —> x = 3 <br>

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——<br>

Check:

3*3 + 6*2 = 9+12 = 21 &#10004;
3*3 + 3*2 = 9+6 = 15 &#10004;