[tex]\bf f(x)=\cfrac{1}{2}x+a\\\\
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\stackrel{x=a}{f(a)}=\cfrac{1}{2}(a)+a\implies 3=\cfrac{1}{2}(a)+a\implies 3=\cfrac{a}{2}+a\implies 3=\cfrac{a+2a}{2}
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6=a+2a\implies 6=3a\implies \cfrac{6}{3}=a\implies \boxed{2=a}\\\\
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f(8)=\cfrac{1}{2}(\stackrel{x}{8})+(\stackrel{a}{2})\implies f(8)=\cfrac{8}{2}+2\implies f(8)=6[/tex]
