The Resultant Force

By Newton's Second Law,

the acceleration of an object is proportional to the net force acting on the object.

The net force is the vector sum of all forces acting on the object.

The object will start moving at a 45 degree angle and thus the net force acts at a 45 degree angle.

From the theorem of Pythagoras,

F(R) = ( ( 100 N )^2 + (100 N) ^2 ) ^ 1/2 = 141 N.