Answer: (x, y) = (-26, 10)
This means x = -26 and y = 10 pair up together
===============================================================
How to get that answer:
Add the equations straight down. Think of having 3 separate columns for the three different like terms.
- 3x + (-3x) becomes 0x and that's just 0. So the x terms go away
- 9y + (-6y) is the same as 9y-6y which turns into 3y
- 12 + 18 becomes 30
After all that, we're left with the simpler equation 3y = 30. That solves to y = 10 after dividing both sides by 3.
Plug this back into any equation involving x and y. Solve for x
3x+9y = 12
3x+9(10) = 12
3x+90 = 12
3x = 12-90
3x = -78
x = -78/3
x = -26
We have found that x = -26 and y = 10 pair up together to form the solution.
-------------------
Checking the answer:
Plug (x,y) = (-26, 10) into the first equation
3x+9y = 12
3(-26)+9(10) = 12
-78+90 = 12
12 = 12
We get a true equation since the same thing is on both sides.
Repeat the same idea for the second equation
-3x-6y = 18
-3(-26) - 6(10) = 18
78 - 60 = 18
18 = 18
The second equation is true as well.
Both equations are true, so the solution (x, y) = (-26, 10) is confirmed.
