That is indeed a function. How do you define a function? That's easy! It is simply an equation in which you can arrange it to have one variable standing alone on one side of the equals sign and the other variable on the opposite side. A function must also have the ability to be graphed. Your equation is indeed a function and could be written as either
y = -x + 2 or
x = -y + 2
either way it is a function! Hope that helps!