$\displaystyle \int^1_{-1} t^3 (1+ t^4)^3 dx $

How do I go about solving this one? There are t's but it has dx

I made $\displaystyle u = 1 + t^4$

but $\displaystyle \frac {du}{dt} $ isn't equal to $\displaystyle 4t^3$, right?

What do I do?

Thanks, chickeneaterguy