There are a few ways to solve these, but the substitution method is often easiest.
1. Substitute 2x in for y in the 1st equation: x + (2x) = 6.
(This can be done because the 2nd equation says y and 2x are interchangeable).
2. Simplify: 3x = 6.
3. Solve for x: x = 2.
4. Plug 2 in for x in the 1st equation: 2 + y = 6.
5. Solve for y: y = 4.
6. Final Answer: (2, 4). (x = 2, y = 4).
That's the substitution method - the next example has a few more steps:
2x-3y=-13
y=2x+7
1. Substitute 2x + 7 in for y in the 1st equation: 2x - 3(2x + 7) = -13.
(This can be done because the 2nd equation says y and 2x + 7 are interchangeable).
2. Simplify: -4x - 21 = -13.
3. Solve for x: x = -2.
4. Plug -2 in for x in the 1st equation: 2(-2) - 3y = -13.
