Given $x^3+x^2y+xy^2+y^3=81$
This is not a standard differentiation of the form $y = f(x)$. We need to differentiate each term separately on LHS and RHS and then calculate $\large \frac{dy}{dx}$
According to the Product Rule for differentiation, given two functions $u$ and $v, \large \frac {d(uv)}{dx} $$= u \large \frac{dv}{dx}$$+ v \large \frac{du}{dx}$
Differentiating both sides:
$\Rightarrow 3x^2\; dx + 2xy \; dx + x^2\; dy + dx\; y^2 + 2xy \; dy + 3y^2\; dy = 0$
$\Rightarrow (x^2+2xy+3y^2)\; dy = -(3x^2+2xy+y^2)\; dx$
$\Rightarrow \large \frac{dy}{dx} $$= -\large \frac{y^2+2xy+3x^3}{x^2+2xy+3y^2}$