I'm having a spot of trouble with this problem:

Find the surface area generated by rotating the given curve about the y-axis.

x=(e^t)-t

y=4e^(t/2)

0<t<8

My homework is submitted online, and I keep working this problem out but it says I have it wrong, but I'm pretty sure I'm doing it right.

I know A(s) = integral from 0 to 8 of 2pi x(t)sqrt((dx/dt)^2+(dy/dt)^2)dt, but I keep messing up. Help!

...Maybe i'm using the wrong equation?