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> Let $f(x,y) = \frac {-1}4 (3xy^2 - 5x^3y + 2x^4)$. Find the equation of the tangent plane to $f$ at the point $(2,4)$.
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Using vector dot product with:

$a = 2$  

$b = 4$  

$f(a,b) = -8$  

$\frac {\partial f} {\partial x}(a,b) = -56$  

$\frac {\partial f} {\partial y}(a,b) = -22$

I worked out the answer:

$56x + 22y - 2 = 192$

but according to the solution sheet this is wrong. Would really appreciate help on where I am going wrong.

Thanks!