find $\displaystyle d^2y/dx^2$ by implicit differentiation

$\displaystyle 3x^2-4y^2=7$

$\displaystyle 6x-8yy'=0$

$\displaystyle y'=\frac{6x}{8y}=-\frac{3x}{4y}$

$\displaystyle \frac{d^2y}{dx^2} = \frac{3x(4y')-4y(3)}{16y^2}$

i don't see this moving towards $\displaystyle -\frac{21}{16y^3}$which is the answer