1)
11x+5y=20
2x+5y=-25
multiply the second equation by - 1 and add the result with the first equation:
11x + 5y = 20
- 2x - 5y = 25
-------------------
9x = 45
=> x = 45 / 9
=> x = 5
Replace in 2x+5y=-25
=> 2(5) + 5y = - 25
5y = - 25 - 10
5y = - 35
y = - 35 / 5
y = - 7
2)
3x+7y=-21
3x-7y=63
Add the two equations:
3x + 3x + 0 = - 21 + 63
6x = 42
=> x = 42 / 6
=> x = 7
Replace in 3x-7y=63
3(7) - 7y = 63
=> 7y = 21 - 63
=-> 7y = - 42
=> y = - 42 / 6
=> y = - 7
