PHYSICS---am i doing anything wrong?

Starting speed is 30 m/s

End speed is 15 m/s

Time elapsed is 3 s

Find the distance traveled (acceleration is constant)

the formula they gave me is

$\displaystyle x-x^0=1/2(v^0+v)t$

just pretend the $\displaystyle ^0$ is below not above, i dont know how to do sub-0 sorry.

anyway, i plug it all in and get

$\displaystyle

1/2*(15+30)*3=67.5 m

$

But the book says the answer is 90 m(Headbang)

thanks to any1 who helps! i think the book may be wrong, it wouldn't be the first time, but I'd like to be sure.