The cosine rule applies to any triangle. You don't need a right angle.
a² = b² + c² - 2bc cos A <- this version is used to find the missing side
cos A = b² + c² - a² / 2bc <- is used to find the missing angle.
Example;
pq = a b=8cm c=15cm angle PRQ = 70
a² = b² + c² - 2bc cos A
PQ² = 8² + 15² - 2 X 8 X 15 X cos 70
=206.9151..
PQ= 14.4 cm (to 3 sf)
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