This question was uploaded on 27/11/21 on social media accounts.
Solution:
Two triangles are congruent in the above figure:
By cosine rule:
\[\cos120 = \frac{5^2+3^2-x^2}{2.5.3}\]
\[\Rightarrow x=7\]
Side = 5+x+3 = 15
Now area:
\[A = \frac {bh}2 = \frac{7\times\frac{15\sqrt3}{2}}2 = \frac{105\sqrt3}4\]
