4. The temperature, in degrees Celsius, at a point (x, y) on a flat plate is given by f(x, y) = 3x - 2y2, where x and y are measured in feet. Let P be the point (1, 1) on the plate. (a) Find the rate of change of temperature at P in the direction toward the point Q(4,5). (b) In which direction does the temperature decrease most rapidly at P? What is that rate of change?