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[tex]\boxed{\begin{array}{l}\sf dada~uma~func_{\!\!,}\tilde ao~f(x)=ax+b\\\sf sua~ra\acute iz~\acute e~dada~por~x=-\dfrac{b}{a}.\end{array}}[/tex]
[tex]\Large\boxed{\begin{array}{l}\sf f(x)=3x-15\\\sf a=3~~b=-15\\\sf x=-\dfrac{b}{a}\\\sf x=-\dfrac{-15}{3}\\\sf x=5\\\huge\boxed{\boxed{\boxed{\boxed{\sf\maltese~alternativa~b}}}}\end{array}}[/tex]