If $y=\log \left(x+\sqrt{x^2+a^2}\right)$, then $\frac{d^{2}y}{d{x}^2}$ is equal to

If $y=x^2+2x+3$, then $\frac{d^{2}x}{d{y}^2}=$

If $x=f\left(t\right), y=g\left(t\right)$, then $\frac{d^{2}y}{d{x}^2}$ is