Using the definition of derivative of a function at a point, $f'(a) = \lim_{h \to 0} \frac{f(a+h) - f(a)}{h}$, find the slope of the curve at the given points or state that the slope is undefined.
$f(x) = \frac{-6}{-7x+4}$
At $a = 1$, the slope is
At $a = \frac{4}{7}$, the slope is