rewriting (a) as
$y = -(x)^{\frac 1 2} + x^{-\frac 4 7}-4x^{-3}$
might make finding $\dfrac{dy}{dx}$ a bit easier. Just apply $\dfrac{d}{dx} x^a = a x^{a-1}$
for (b) I'd go ahead and multiply through to obtain
$y = x^{\frac 5 2} - x,~0 \leq x$
note the condition is required on the right hand side as the square root function is only defined for non-negative real numbers
Now you can easily apply the differentiation formula above.