A 1.2 kg coffee cup is accelerated at 3.0 m/s^2 in the direction shown, over a frictionless horizontal surface. The acceleration is caused by three forces, but only two of which are shown. The magnitudes of the given forces are as follows: F1= 10N and F2= 20N. Find the magnitude and direction (with respect to the positive x axis) of the third force, F3.