Answer
$\approx 0.778$
Work Step by Step
$D_uf=f_x(x,y)a+f_y(x,y)b$
$D_uf=\dfrac{-39-(-26))}{-25-(-15)} \times \dfrac{1}{\sqrt2}+\dfrac{-34-(-30))}{40-20} \times \dfrac{1}{\sqrt2}$
This implies
$D_uf=\dfrac{13}{10} \times \dfrac{1}{\sqrt2}+\dfrac{-1}{5} \times \dfrac{1}{\sqrt2}$
$D_uf \approx 0.778$
