6x - 5y = 5
3x + 5y = 4
If the left sides are equal to the right sides then if we add left side of first equation to left side of the second one and right side of the first euation to right side of the second one they still would be equal
6x - 5y = 5
3x + 5y = 4
9x +0y = 9
9x = 9
x = 1