Problem with trigonomic derivative (pretty simple)

I know how to derivate sinx, cosx and standard equations like

y = x^4 + sqrt.(x)

y' = 4x^3 + x^-1/2

y = sinx + cosx

y' = cosx - sinx

and so on. But now they want me to "decide an equation for the tangent to the curve":

a) y = cosx (pi/2, 0)

Answer: y = -x + pi/2

b) y = 2x - 5sinx (pi, 2pi)

Answer: y = 7x -5pi

I'm new to derivatives with sinx, and they have almost no examples in the book unfortunetly. I have almost no idea how to solve this, except that I guess pi would stand for 180 degrees, so then pi/2 would be x = 90 degrees. After that I'm lost!

Appreciate help!!