تابع خطی                                                     $f(x) = ax + b$ $a(2x + 1) + b = a(x - 1) + b + a(x + 1) + b - 7$$\begin{array}{*{20}{c}}{a = b - 7\,\,\,\,\,\,\,\,\,}\\{9 =  - 2(a) + b}\end{array} \Rightarrow \left\{ {\begin{array}{*{20}{c}}{a - b =  - 7}\\{ - 2a + b = 9}\end{array}} \right.$$ \Rightarrow  - a = 2 \Rightarrow a =  - 2\,\,\,,\,\,\, - 2 - b =  - 7 \Rightarrow 5 = b$$f(x) =  - 2x + 5 \Rightarrow f(4) =  - 2(4) + 5 =  - 8 + 5 =  - 3$