From the simple interest equation, I=PRT, you're solving for R. The answer will be in decimal form. Multiply by 100 to get the percent interest. First though, I is not the total amount you have after 6 years ($6200) but the amount you earned ($1200).
1200 = 5000...
Multiply 25.40 by .058 to get your answer. 25.40*0.058 = 1.4732 or $1.47.
The best way to answer this question is with the sum formula for cosine. cos (a + b) = cos a * cos b - sin a * sin b cos (7pi/12) = cos (3pi/12 + cos 4pi/12) = cos (pi/4 + pi/3) Good luck and let me know if you still need help!