Hello, this question is just a simple application of three new triangle formulas that is introduced in Sec 3 Math - namely cosine rule, sine rule, and a new formula for the area of a triangle. Note that these 3 formulas can be applied to ALL triangles, as opposed to Pythagoras Theorem or TOACAHSOH trigonometric rules that you have learnt in Sec 2 (which only applies to right angled triangles). Hence in this case, our Sec 2 triangle formulas cannot be used, and these 3 formulas are the formulas that you should use to help you solve the problem instead. Leave a comment if you still need clarifications / help!