Question 1168869
It is a function.
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The test is to graph it and then slide a vertical line along that graph.  If the vertical line intersects the graph of f(x)=-x at most once then the equation is a function.  The key here is that for each possible input x, there is one corresponding value of f(x).  Here is a graph of f(x)=-x with a vertical line for illustration, sliding the line left or right you can see it will only intersect f(x)=-x once:
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{{{ drawing(400,400,-10,10,-10,10, grid(0), 
line(-10,10,10,-10), 
line(3,10,3,-10),
locate(-8,9, "f(x) = -x")

) }}}