Quick convolution integral checking

[dfx]

Homework Statement

Consider a linear system with the impulse response:

g(t) = 3x^2 - 4x + 7 for t>0 and 0 otherwise.

Find the output for the input f(t) = t for t \geq 0 and f(t) = 0 for t<0.

Homework Equations

\[ \int_{-\infty}^t f(t - \tau)g(\tau)\,d\tau\]

The Attempt at a Solution

\[ \int_0^t f(t - \tau)g(\tau)\,d\tau\]

\[ \int_0^t (t - \tau)(3\tau^2 - 4\tau + 7\,d\tau\)]

and the answer I keep getting is

\frac{t^4}{4} - \frac{2t^3}{3} + \frac{7t^2}{2}

whereas the official given answer has the sign in the middle term as a plus: +\frac{2t^3}{3}

I've even tried wolfram and I think I'm correct:

http://img58.imageshack.us/img58/8637/mspzk2.gif (obviously with different variables - x instead of tau, but still evaluted between t and 0).

If anyone could clear up the correct answer that would be much appreciated.

 

[dfx]

...anyone?

 

[Vagrant]

Even I got the same answer as you. So I guess not much of a help.