x = 0 and y = 4
It is a linear equation in x. The equation has the solution x = 0.
The solution: x = 6/7 and x = -1
The equation 6 + y = 12 is a simple equation in one variable. If you add 6 to both sides of the equals sign, you get y = 18, and that is the solution.
It's the equation of a straight line. Every point on the line is a solution to the equation. There are an infinite number of them.
There is no solution since the equation is inconsistent.
x = 0 and y = 4
The equation does not have a solution.
-5
Without an indication of the equals sign, this is not an equation.
Not sure what you mean; that equation certainly has a solution.
It is a linear equation in x. The equation has the solution x = 0.
x = 19
One equation with two unknowns usually does not have a solution.
That is not a correct equation. The numbers on the left equal -16, not 0. This is an equation with no solution.
The solution is (x, y, z) = (2.5, 1, 2.5).
21 is the only solution of that equation.