[tex]\displaystyle\bf\\A(1,1) \in G_f \implies f(1)=1.\\B(3,5) \in G_f \implies f(3)=5.\\asadar,~folosindu-ne~de~ce~am~scris~mai~sus \implies \\\left \{{{a+b=1} \atop {3a+b=5}} \right. \Leftrightarrow \left \{{{a=1-b} \atop {3(1-b)+b=5}} \right. \\3-3b+b=5 \Leftrightarrow -2b=2 \implies b= -1,~deci~a=2.\\f(x)=ax+b=2x-1.\\f(x)=7\Leftrightarrow 2x-1=7 \Leftrightarrow 2x=8 \implies x = 4.[/tex]
