how do i find theta??

Question

What angle does Fr make with the positive x axis?
Fr=1.8 N
Three forces of magnitudes F1=4.0N,F2=6.0N , and F3=8.0N are applied to a block of mass , initially at rest,
F1 is at an angle of 25 degrees
F2 is at an angle of 10 degrees(in the - y direction)
F3 is at an angle of 235 degrees

Answer 1

Add the x and y components for each force,
f1x= F1cos(25d); f2y = F1sin(25d)
etc.
frx=f1x+f2x+f3x
fry=f1y+f2y+f3y
theta=arctan(fry/frx)
[or better, theta = arctan2(fry, frx) which accommodates the full 360-deg range of angles whereas the normal arctan function can only output angles in the +x half of the plane, -90 to 90 deg. But in this case it's unnecessary since the problem was set up for a +x resultant.]
EDIT: Steve's numbers are suspect. I have
f1 = (+3.625,+1.690)
f2 = (+5.909,-1.042)
f3 = (-4.589,-6.553)
fr = (+4.945,-5.905)
mag = 7.702, angle = -50.052 deg

Answer 2

I'm assuming that 10° in the -y direction means -10°........

First, find Fr (It's not 1.8 N)
Do this by summing the 3 x components = Σx and the y components ( Σy) Fr will then = √(Σx² + Σy²) = 7.946 N

Θ = arctan(Σy/Σx) = -48°