How to find the missing angle of a triangle using trig when you only know 1?

If you have

2 KNOWN sides & 1 Angle ;
Use Cosine Rule

If you have 3 KNOWN sides,
Also Cosine Rule

If you have

2 KNOWN angles & 1 side,
Use SINE rule

If it's a right-angled triangle, use SOH CAH TOA

Hopefully this helps