You should have a list of formulas. Very few can get through trigonometry without it. I recall, in my first trigonometry book, the magic page was Page 89. Without Page 89, few could survive an examination. Find such a list in your materials and hang it from your hat until it soaks in. Paste a copy on your bathroom mirror - maybe one on the refrigerator.
There are six formulas like this. Find them all.
sin(a-b) = sin(a)cos(b)-cos(a)sin(b)
It's not magic. It requires memorization.
I am not being harsh. I am being honest. Listen and learn.
Note: "pie" is for eating. "pi" is for math and Greeks.