TL:DR: 29.4 m/s^2
The equation for the acceleration due to gravity is:
g = G * (M/r^2)
Where G is the gravitational constant, M is the mass of the planet (or other body) and r is the radius of the planet (or other body).
(Note: This equation can be found by making a force equation for an object, where the only force acting on it is gravity, or in other words, the object is in free fall. The mass of the object cancels out, since it is in the numerator on both sides, leaving you with the above equation.)
Now, you could track down the values for the radius and mass of the earth, and then enter it all into this equation with the corrections for the problem. But we know what the equation gives us for Earth (9.8 m/s^2), so we just adjust that answer accordingly.
The mass of this new planet is 12 times that of Earth, which will multiply the acceleration by 12, and the radius is 2 times that of Earth, which will divide the acceleration by 4 (since r is squared).
So we have the new acceleration (which we will just call a) is equal to:
a = (12 * g)/ 4 = 3 * g = 3 * 9.8 = 29.4 m/s^2.
In other words, things would accelerate very quickly.
