Đáp án:
$\begin{array}{l}
f\left( x \right) = \left| {x - 4} \right| + 7\\
a)f\left( { - 1} \right) = \left| { - 1 - 4} \right| + 7 = \left| { - 5} \right| + 7 = 5 + 7 = 12\\
f\left( 3 \right) = \left| {3 - 4} \right| + 7 = \left| { - 1} \right| + 7 = 1 + 7 = 8\\
f\left( 4 \right) = \left| {4 - 4} \right| + 7 = \left| 0 \right| + 7 = 0 + 7 = 7\\
b)\\
+ )f\left( x \right) = 7 \Rightarrow \left| {x - 4} \right| + 7 = 7\\
\Rightarrow \left| {x - 4} \right| = 0 \Rightarrow x = 4\\
+ )f\left( x \right) = 8\\
\Rightarrow \left| {x - 4} \right| + 7 = 8\\
\Rightarrow \left| {x - 4} \right| = 1\\
\Rightarrow \left[ \begin{array}{l}
x - 4 = 1\\
x - 4 = - 1
\end{array} \right. \Rightarrow \left[ \begin{array}{l}
x = 5\\
x = 3
\end{array} \right.
\end{array}$
