Solve the Problem $M(x, y)=5 x^{2}+45 y^{2}-2 x y$ , Where $x$

Multiple Choice

Solve the problem.
-Suppose that the manufacturing cost of a precision instrument is approximated by $M(x, y) =5 x^{2}+45 y^{2}-2 x y$ , where $x$ is the cost of materials and $y$ is the cost of labor. Find $\left(\frac{\partial \mathrm{M}}{\partial \mathrm{y}}\right) (8,4)$ .

A) 72
B) 256
C) 344
D) 656