I'm not sure what I need to do. I know how to do it with numbers but not letters and shapes like this​

It would be a/c

Step-by-step explanation:

Since you're solving for angle F, you have to look at it from the angle F's perspective. Sin is opposite/hypotenuse. I recommended labeling the sides with H, O and A to make it less confusing. The letter opposite angle F is a, so you can label that "O". The side adjacent to angle F is b, so label that side "A" and the side c can be labeled as "H". Since sin is O/H, the correct answer would be a/c.