You can solve this system of equations in many ways.
This easiest way, in my opinion, is by addition.
10x+15y=20
+
-9x-15y=-21
---------------------
x=-1
Now you have x = -1
plug that into any of the equations to get y
You can also solve this with substitution as well.
10x+15y=20
-9x-15y=-21
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