X plus y equals 4

Answer 1

x + y + x = 2x + y

Answer 2

Solving equations in two unknowns requires two independent equations. Since you have only one equation there is no solution.

Answer 3

it is a linear equation in 2 variables

Answer 4

They can be but need not be.

Answer 5

(0, ), (2, ), ( , 0)

4 2 4