[tex]Subtitute\ x=\dfrac{m}{4}\ to\ f(x)=8x^2-4x:\\\\f\left(\dfrac{m}{4}\right)=8\left(\dfrac{m}{4}\right)^2-4\left(\dfrac{m}{4}\right)=8\cdot\dfrac{m^2}{16}-4\cdot\dfrac{m}{4}\\\\\boxed{f\left(\frac{m}{4}\right)=\frac{m^2}{2}-m=\frac{m^2-2m}{2}}[/tex]
