# How do you evaluate the function f(x) = 4x-5  for f(3)?

Replacing $x$ with $3$ in $4 x - 5$ tells us $f \left(3\right) = 7.$
To evaluate $f \left(3\right) ,$ just replace all $x$ in $f \left(x\right) = 4 x - 5$ with $3 :$
$f \left(3\right) = 4 \left(3\right) - 5 = 12 - 5 = 7$