SOLUTION: Sin420°cos390 - cos(-300°)sin(-330°)

sin(420°)*cos(390°) - cos(-300°)*sin(-330°) = 


     sin(420°)  = sin(60°);
     cos(390°)  = cos(30°);
     cos(-300°) = cos(60°);
     sin(-330°) = sin(30°).


= sin(60°)*cos(30°) - cos(60°)*sin(30°) = sin(60°-30°) = sin(30°) = .