Suppose that Henry's tastes over peanuts (x) and potato chips (y) are summarized by the utility function  u(x,y) = x+y and and p_x = 20, p_y = 30 and m = 300. Now let's suppose that the price of peanuts (p_x) suddenly increases to p_x = 35 while income stays the same. Calculate a change in demand for peanuts due to the substitution effect.