Answer:
b² = a² + c² - 2ac cos B
Explanation:
The cosine rule can be use when you are given three sides of a triangle and ask to find any angle or when you are given two sides and a included angles(the angle between the two sides given) and ask to find the length of a side.
The equation Sophia can use to solve for the side length(b) of a triangle when given two length and the included angle can be expressed below. Sophia was given two sides and an included angle and was asked to find a side length b.
For sides a, b and c the cosine rule can be represented below
a² = b² + c² - 2bc cos A
b² = a² + c² - 2ac cos B
c² = a² + b² - 2ab cos C
The equation required base on your question is
b² = a² + c² - 2ac cos B