Answer: to east.
Explanation: Momentum is the "amount of movement" and can be written as:
P = m*v
where P is momentum, m is the mass of an object moving and v is the velocity of the object.
As you may know, the velocity is a vector and the mass is a scalar, so the momentum must be also a vector, that has the same direction that the vector of velocity, so if the velocity of the rock points to the east, also does the momentum.
