1999c2…: “As well as the bright set”

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Suppose that $f(x,y)$ is differentiable and $\grad {f}(1,2) = \vector {1,3}$ . If $f(1,2)=4$ , find the equation of the tangent plane to the surface $z=f(x,y)$ at the point $(1,2,4)$ .

$x + \answer {3} \cdot y + \answer {-1} \cdot z = \answer {3}$