I'm just getting into derviatives as a rate of change.

The equation of a moving body is given by 3t^2 - 5t + 7 feet.

My book says that the velocity is the derivative of this equation and that is 6t-5

However I got something completely different when I derived the first equation.

Then the derivative of 6t-5 at t=3 ends up being 6. But I still don't get that.

What am I doing wrong?