Can someone give me some guidance please, just started new calculas assignment and need some help:

If f(theta) = 10Sin^2(0.4theta - 0.1)

Show that f'(theta) = 4Sin(0.8theta - 0.2)

and evaluate f''(pie/4) to 3 decimal places?

For first part do I first change sin to cos and then remove the brackets which would give me 4Cos^2 but not much help with the rest?

I should get further than this but just need some help getting started.

Any help much appreciated.