Answer:
84.9°
Step-by-step explanation:
You correctly found the acceleration and the net force in the x direction. Now write the net force as the sum of the x components of the individual forces.
∑F = F₁ + F₂ cos θ
3.6 = 2.8 + 9.0 cos θ
0.8 = 9.0 cos θ
0.0889 = cos θ
θ = ±84.9°
F₂ is either 84.9° above the x-axis or below the x-axis.