OK. Hold on tight. Here we go.
Remember ... the "solution" is the number that 'g' must be
in order to make the equation a true statement.
The original equation: You said that -2 = g - 9
Now, let's add 9 to each side: 7 = g .
The surface (call it
) is a triangle with vertices at the points



Parameterize
by

with
and
. Take the normal vector to
to be

Then the flux of
across
is



Gotta be 40, since the sample probability of choosing a pink was 2/5, apply that to 100 chips and you get a predicted 40 pink chips in the bag