7. The derivative ∇_u f(a) of the function f(x, y, z) = 3x²y + 2y³z² − x³z² + xy - 12 in the direction
u = v/||v|| unde v = =(2, - 1, - 2) at the point a = (1, 1, 3) - is equal to (fill in the obtained value)