Calculating Force F3: Magnitude & Direction

[teh81]

Homework Statement

Three concurrent forces, F1, F2 and F3, act on a block that is placed on a
horizontal surface. The resultant force on the block is 20.0 N at 0°. If F1 = 10.0 N
at 40° and F2 = 20.0 N at 180°, calculate the magnitude and direction of the force
F3. Sketch a vector diagram (not to scale) illustrating your solution.

 

[ideasrule]

What have you tried so far?

 

[teh81]

i drew the forces on the block.
then i found the f1 on x and y component ..
f1x= 10cos40 = 7.67N
f1y= 10sin40 = 6.43N
i know that the sum of all forces will give me 20n, is that right?
and .. the f2 is pushing the block in side < ----
but I am not sure how find the f3 on x and y component..
if i found it,
i know that i have to plug in
f3=sqrt(f3x^2+f3y^2)