Translational Equillibrium, how do i show my work?

[Senjai]

12. Three horizontal forces are acting at the same point on a puck lying on a level frictionless
table. The puck is in translational equilibrium.
Two of these forces are:
F1 = 10.2 N, east
F2 = 15.4 N, 30° E of N
What is the magnitude of the third force?

I can figure out the answer quite easily, I add F1 and the horizontal component of F2 together, and use Pythagorean theorem to find the resultant of the new Fx component and F2y to find the resultant, and the force to place it in equilibrium would be the negative of that vector.

How do i show my work for this.. I am sure i can't just say its the opposite of the net force.

 

[rl.bhat]

If F(R) is the resultant of F1 and F2, for equilibrium F3 + F(R) = 0.
Or F3 = - F(R)

 

[Senjai]

thank youu ! that'd make sense. :)