Below what pH will free phenobarbital begin to separate from a solution having an initial concentration of 4 g of sodium
phenobarbital per 100 mL at 25°. The molar solubility of
phenobarbital is 0.0050 and the pKa is 7.41 at 25°C. The molecular
weight of sodium phenobarbital is 254

because it says use -2,0,and 2 for x, we can plug those numbers into the equation. y/4=3/4(-2)+2   y/4= -1 1/2 +2 now to get x by itself, we can multiply each side by 4. y=-6+8 which equals y=2 when x=-2. now you can do the same thing with 0 and 2 as x. you get y=8 when x=0 and y=14 when x=2.

x   |   y

-2 |   2

0   |   8

2   |   14

answer( sicar );