Implicit Derivative Equation

Find the slope of the tangent to the curve x^2 + xy + y^2 = 7 at (1,2).

I got that the derivative was -(2x+y)/(xy+2y), then I plugged everything in and got -4/6, but Webwork is telling me it's wrong. Help is much appreciated!

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x² + xy + y² = 7

        2x + xy'+y·1 + 2yy' = 0            ← now, substitute x=1 and y=2
2(1) + 1y' + 2·1 + 2(2)y' = 0
              2 + y' + 2 + 4y' = 0
                                 5y' = -4
                                   y' = -4/5            ← ANSWER


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2x+(y+x y')+2y y' = 0

(y+x y')+2y y' = -2x

y' (y+x+2x)=-2x

y' (y+3x)= -2x

y' = -2x/(y+3x)

y' = -2 / 5


(y' = dy/dx = slope of tangent)