Solve for x

1. $\displaystyle sin3x= \frac{3}{4}$

2. $\displaystyle 4sinx.cosx.cos2x=1$

3. $\displaystyle -cos4x=2$

4. $\displaystyle cos(x+\frac{pi}{18})=\frac{2}{5}$

Are there some certain ways to solve trig problems like these? Please help me. Thank you