Answer
$f(a+4)=-3a-8$
Work Step by Step
To find the value of $f(a+4)$, substitute $a+4$ to $x$ in $f(x)$ to obtain:
$f(x)=-3x+4
\\f(a+4)=-3(a+4)+4
\\f(a+4) = -3(a) +(-3)(4)+4
\\f(a+4)=-3a+(-12)+4
\\f(a+4)=-3a+(-8)
\\f(a+4)=-3a-8$
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