et the first column by holding $y$
    constant, and then differentiating with respect to $x$. We call
    this the <em>partial derivative</em> of $f$ with respect to $x$.
    We'll write this as $\frac{\partial f}{\partial x} = 2xy+3$ or
    $f_x = 2xy+3$.</li>

    <li>The second column of the derivative is the <em>partial
    derivative</em> of $f$ with respect to $y$&mdash;it tells us how
    changes in $y$ affect the output. We can get the second column
    by holding $x$ constant and differentiating w