What is the Magnitude of the Third Force in a Three-Force System?

[purplecows]

Homework Statement

An object that is moving at a constant speed velocity is acted upon by three forces. One force is 150 N along the x-axis, the second is 100 N along a direction making a counterclockwise angle of 120° with the x-axis. What is the magnitude of the third force?

Homework Equations

Not sure, these are the only ones I THINK are remotely relevant (but I have no idea which is why I'm posting here)

F_x = Fcosθ
F_y = Fsinθ

The Attempt at a Solution

I thought it would just be either 100 N (the same as the northwest force 120° from x-axis) or it would be something involving the Pythagorean Theorem (100^2 + f^2 = 150^2 or 100^2 + 150^2 = 150^2)


Anyone? This is urgent. My whole class is confused.

 

[Pythagorean]

Since it's moving at a constant velocity, what is the sum of the forces on the object?

 

[purplecows]

Since it's moving at a constant velocity, what is the sum of the forces on the object?

zero

 

[Pythagorean]

And the x and y directions are independent. So you have two equations, two variables, yeah?