Factorizing -Where to next?

Hi everyone, having a lot of trouble with Calculus and was wondering if anyone can help.

I have d(t)= -5e^(-5t) x cos(10t)

u = -5e^(-5t)

v = cos(10t)

du/dx = 25e^(-5t)

dv/dx = -10sin10t

duv/dx = 25e^(-5t) x cos(10t) + (-10sin10t) x -5e^(-5t)

= 25e^(-5t)cos(10t) + 50e^(-5t)sin10t

= 2cos10t+sin10t

From here I am not sure where to go? Do I have the first bit correct?

0=(sin+cos=1)

0=2 x 1 x 10t

0=20t

t=1/20 ???

Or do i have to find out tan = sin(a) / cos (a)

If anyone could give me some kicks in the right direction it would be much appreciated, and i can stop wasting paper.(Speechless)

Thanks

Kayne