My guess is that you're supposed to let sin(a - b) = sin(a + (-b)), and then use the fact that sin(-x) = -sin(x).
Given this formula
Sum
sin(a+b)=sin(a)*cos(b)+cos(a)*sin(b)
prove this one
Difference
sin(a-b)=sin(a)*cos(b)-cos(a)*sin(b)
Given this formula
Sum
cos(a+b)=cos(a)*cos(b)-sin(a)*sin(b)
prove this one
Difference
cos(a-b)=cos(a)*cos(b)+sin(a)*sin(b)
I assume it's the same concept for both of them, i just don't know how to go about proving it to be true.