$a)F\cdot \cos \alpha =1200\cdot \cos {20}^o=1200\cdot 0,9397=1127,64$  

 

$b)F\cdot \sin \alpha =120\text{N}$  

$300\text{N}\cdot \sin \alpha =120\text{N}$  

$\sin \alpha =\frac{120}{300}=0,4$  

$\alpha \approx {24}^o$