The sine and cosine laws
Cosine law when you know two sides and the angle between them. Sine law when you have an angle paired with its opposite side.
A worked example
Two sides 11 and 8 with 48° between: c² = 11² + 8² − 2(11)(8)cos 48°, so c = 8.2.
The mistake people actually make
If the known angle sits between the two known sides, the sine law cannot get you started. Reach for the cosine law.