Okay, do it! Start with a Right Triangle. Label A and B. Lable the legs so that tan(A) = 12/5. Determine the hypotenuse and you are well on your way.
If A= 6B then B= A/6 so 2B= A/3.
Hopefully, you know that sin(x+ y)= sin(x)cos(y)+ cos(x)sin(y) and that cos(x+y)= cos(x)cos(y)- sin(x)sin(y). Taking x= y, sin(2x)= 2sin(x)cos(x) and . Then .
If x= A/3, then
From tan A= 12/5, you can solve for sin(A) and cos(A). Then you have two equations to solve for sin(B) and cos(B).