[tex] f(x) = -mx + m\\ \\ A\left(-1;\dfrac{1}{2} \right) \in Gf \Rightarrow f(-1) = \dfrac{1}{2} \Rightarrow \\ \\ \Rightarros -m\cdot (-1)+m = \dfrac{1}{2} \Rightarrow m+m = \dfrac{1}{2} \Rightarrow \\ \\ \Rightarrow 2m = \dfrac{1}{2} \Rightarrow \boxed{\boxed{m= \dfrac{1}{4}}}[/tex]