Any function is called linear if the power of the unknown terms in the eqaution is 1.
Thus, the given equation x+y = 5 is linear because the power of the unknown terms x & y in the equation is 1.
Any linear equation is of the form "y = mx + b". By subtracting x from each side of the given equation, we get "y = -x + 5", satisfying what is required.
Yes. This equation can be rewritten as y = 5 - x which clearly shows that y is the dependent variable and x is the independent variable.