Answer:
13.3
Step-by-step explanation:
Let the missing side be [tex]x[/tex]. Using cosine rule,
[tex]x^{2} = 18^{2} + 8^{2} -2(8)(18)cos43[/tex]
[tex]x = \sqrt{18^{2} +8^{2}-2(18)(8)cos43 }[/tex]
[tex]x = 13.3[/tex] (3 sf)
