To figure out which of the coordinates don't work, we need to substitute their values of x and y to figure out if they work or not. We will know they don't work if the result we get is something crazy like 2=25.
Substitute the values of x and y from a
[tex] \left \{ {{5 (6) = 3(5) +15 } \atop {6(5)=10(6)-30}} \right. [/tex]
Now we simplify
[tex] \left \{ {{30 = 30 } \atop {30=30} \right. [/tex]
Since this is true we know that the point (5,6) works. Now let's move onto B.
Once again, we substitute.
[tex] \left \{ {{5 (12) = 3(-15) +15 } \atop {6(-15)=10(12)-30}} \right. [/tex]
This simplifies to
[tex] \left \{ {{60=-30} \atop {-90=90} \right. [/tex]
Since this is obviously not right, we know that is the point that doesn't work in this system of equations.
