Hi there!
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I believe your answer is:
[tex]f=-15[/tex]
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Here’s why:
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[tex]\boxed{\text{Solving for "f":}}\\\\\frac{f}{3} =-5\\--------\\\rightarrow\frac{f}{3}*3=-5*3\\\\\rightarrow \boxed{f=-15}[/tex]
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Hope this helps you. I apologize if it’s incorrect.