What is the maximum value of the function f(x)=-3x^2+18x-10

The local or global extrema will occur when velocity, dy/dx, is equal to zero.

dy/dx=-6x+18

d2y/dx2=-6

Since acceleration, d2y/dx2, is a constant negative, when dy/dx=0, y is at an absolute maximum value...

dy/dx=0 only when -6x+18=0, -6x=-18, x=3

And the maximum value is:

f(3)=-3(3^2)+18(3)-10

f(3)=17

Yes because she has 0.500 liters of milk and only needs 0.355