The temperature of a metal plate is given by $f(x,y)=\frac{150}{\sqrt{x^2+y^2+1}}$. Find the rate of change of the temperature at the point $(8,4)$, in the direction towards the point $(7,2)$
Right now I have that $\triangledown f(x,y)=\langle\frac{-150x}{(x^2+y^2+1)^{\frac{3}{2}}}, \frac{-150y}{(x^2+y^2+1)^\frac{3}{2}} \rangle$.
Next, I have $\triangledown f(8,4)=\langle\frac{-400}{243}$, $\frac{-200}{243}\rangle$.
From here I'm not sure what to do with the "direction toward $(7,2)$".