Find thet-value where $\displaystyle \kappa(t)=0.0001m^{-1}$

for this vectorfunction:

$\displaystyle \overrightarrow{r}(t)=\binom{450t}{1200-30t^2-7t^3}, t\in[-5,5]$

I know that I must use this formula:

$\displaystyle \kappa(t)=\frac{x'(t)y''(t)-x''(t)y'(t)}{(x'(t)^2+y'(t)^2)^\frac{3}{2}}$

It's just that I don't know where to begin and what to do. :(