IF the question is as you stated it, "128.35" is impossible because if F is positive F cos x would be negative and if F is negative, F sin x would be negative.
So your equations are F cos(x)= -54.684 and F sin(x)= 69.313?
Then clearly 51.65 degrees cannot be correct. I presume neglecting the negative was your error in doing the calculation and you see now that
128.35 degrees is correct? (Actually, I get 128.27 degrees.)